Frequency-independent spatial processing for wideband MISO and MIMO systems

ABSTRACT

Frequency-independent eigensteering in MISO and MIMO systems are described. For principal mode and multi-mode eigensteering, a correlation matrix is computed for a MIMO channel based on channel response matrices and decomposed to obtain N S  frequency-independent steering vectors for N S  spatial channels of the MIMO channel. N D  data symbol streams are transmitted on N D  best spatial channels using N D  steering vectors, where N D =1 for principal mode eigensteering and N D &gt;1 for multi-mode eigensteering. For main path eigensteering, a data symbol stream is transmitted on the best spatial channel for the main propagation path (e.g., with the highest energy) of the MIMO channel. For receiver eigensteering, a data symbol stream is steered toward a receive antenna based on a steering vector obtained for that receive antenna. For all eigensteering schemes, a matched filter is derived for each receive antenna based on the steering vector(s) and channel response vectors for the receive antenna.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a continuation of patentapplication No. 10/650,295 entitled “FREQUENCY-INDEPENDENT SPATIALPROCESSING FOR WIDEBAND MISO AND MIMO SYSTEMS” filed Aug. 27, 2003,allowed, and assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND

I. Field

The present invention relates generally to data communication, and morespecifically to techniques for performing spatial processing forwideband multiple-input single-output (MISO) and multiple-inputmultiple-output (MIMO) communication systems.

II. Background

A MIMO system employs multiple (N_(T)) transmit antennas and multiple(N_(R)) receive antennas for data transmission and is denoted as an(N_(T), N_(R)) system. A MIMO channel formed by the N_(T) transmit andN_(R) receive antennas may be decomposed into N_(S) independentchannels, where N_(S)≦min {N_(T), N_(R)}. N_(S) spatial channels may beformed by the N_(S) independent channels of the MIMO channel and usedfor data transmission.

For a time dispersive MIMO channel, a signal sent from a given transmitantenna may reach a given receive antenna via multiple signal paths(i.e., propagation paths). These signal paths may include aline-of-sight path and/or reflected paths, which are created when thetransmitted signal reflects off reflection sources (e.g., buildings,obstructions, and so on) and arrives at the receive antenna viadifferent signal paths than the line-of-sight path. The received signalat the receive antenna may thus include multiple instances (i.e.,multipath components) of the signal sent from the transmit antenna. Thedelay spread L of the MIMO channel is the time difference between theearliest and latest arriving multipath components (of some certainminimum energy) for all of the transmit-receive antenna pairs in theMIMO channel.

Time dispersion in the MIMO channel causes frequency selective fading,which is characterized by a frequency response that varies across thesystem bandwidth (i.e., different channel gains for differentfrequencies). The multipath components are associated with differentcomplex channel gains and may add constructively or destructively at thereceiver. Time dispersion and frequency selective fading are moreproblematic for a wideband MIMO system with a wide system bandwidth.

Various techniques may be used to combat frequency selectivity in awideband MIMO channel. For example, a multi-carrier modulation techniquesuch as orthogonal frequency division multiplexing (OFDM) may be used topartition the system bandwidth into multiple (N_(F)) orthogonalfrequency subbands. The wideband MIMO channel may then be viewed asbeing composed of N_(F) flat-fading narrowband MIMO channels, each ofwhich may be decomposed into N_(S) spatial channels. Data may then betransmitted on the N_(S) spatial channels of each of the NF subbands.

For a MIMO system that utilizes OFDM (i.e., a MIMO-OFDM system), thewideband MIMO channel can be characterized with (1) a complex channelgain for each of the N_(F) subbands of each of the N_(T)·N_(R)transmit/receive antenna pairs (i.e., N_(F)·N_(T)·N_(R) channel gains inall) and (2) the noise floor at the receiver. The channel gains andreceiver noise floor may then be used to select the data rate(s) fordata transmission on the N_(S) spatial channels of each of the N_(F)subbands. The channel gains may also be used for spatial processing atthe receiver and possibly the transmitter in order to transmit data onthe N_(S) spatial channels of each of the N_(F) subbands. Thus, for theMIMO-OFDM system, frequency selectivity can be combated by treating thewideband MIMO channel as N_(F) flat-fading narrowband MIMO channels andperforming spatial processing separately for each of the narrowband MIMOchannels. However, this frequency-dependent spatial processing cangreatly increase computation complexity at the transmitter and receiver.Moreover, the receiver may need to provide a large amount of feedbackinformation (e.g., the channel gains) to the transmitter to supportfrequency-dependent spatial processing.

There is therefore a need in the art for techniques to more efficientlyperform spatial processing in a wideband MIMO system.

SUMMARY

Techniques for performing frequency-independent eigensteering in MISOand MIMO systems are provided herein. Eigensteering refers to spatialprocessing performed on a data symbol stream with a steering vector at atransmitter in order to transmit the data symbol stream on a spatialchannel of a MISO channel or a MIMO channel. The MISO channel may becharacterized by either (1) a sequence of time-domain channel impulseresponse vectors for a plurality of time delays or (2) a sequence offrequency-domain channel frequency response vectors for the N_(F)subbands. Similarly, the MIMO channel may be characterized by either asequence of channel impulse response matrices or a sequence of channelfrequency response matrices. The eigensteering is frequency-independentin that one steering vector is used for the data symbol stream even ifthe MISO or MIMO channel is time dispersive and regardless of whetherthe eigensteering is performed in the time domain or the frequencydomain. Eigensteering may be performed on one or multiple data symbolstreams with one or multiple steering vectors to transmit the datasymbol stream(s) on one or multiple spatial channels. Variousfrequency-independent eigensteering schemes are described hereinincluding principal mode eigensteering, multi-mode eigensteering, mainpath eigensteering, and receiver eigensteering.

For principal mode and multi-mode eigensteering, a correlation matrix iscomputed for the MIMO channel based on the channel (impulse orfrequency) response matrices for the MIMO channel, as described below.The correlation matrix is then decomposed (e.g., using eigenvaluedecomposition) to obtain N_(S) frequency-independent steering vectorsfor N_(S) spatial channels of the MIMO channel. For principal modeeigensteering, one data symbol stream is transmitted on the principal orbest spatial channel using the steering vector v_(pm) for the bestspatial channel. For multi-mode eigensteering, N_(D) data symbol streamsare transmitted on the N_(D) best spatial channels using N_(D) steeringvectors V_(mm) for these spatial channels, where N_(S)≧N_(D)>1 in thiscase.

For main path eigensteering, a data symbol stream is transmitted on theprincipal spatial channel for the main propagation path of the MIMOchannel using a frequency-independent steering vector v_(mp). For thisscheme, the energy of each channel impulse response matrix is firstdetermined. The main path is the time delay of the channel impulseresponse matrix with the highest energy. A correlation matrix of thechannel impulse response matrix with the highest energy is computed anddecomposed to obtain the steering vector v_(mp) for the best spatialchannel of the main path. The data symbol stream is transmitted on thisspatial channel using the steering vector v_(mp).

For receiver eigensteering, a data symbol stream is steered toward anindividual receive antenna based on a frequency-independent steeringvector v_(rx,i) obtained for that receive antenna. The MIMO channel maybe viewed as being composed of N_(R) MISO channels for the N_(R) receiveantennas. A correlation matrix may be computed for each MISO channelbased on its sequence of channel (impulse or frequency) response vectorsand decomposed to obtain a steering vector for the main spatial channelof that MISO channel. N_(R) frequency-independent steering vectorsV_(rx) may be obtained for the N_(R) MISO channels. N_(D) data symbolstreams may be transmitted using the N_(R) steering vectors V_(rx),where min{N_(R), N_(T)}≧N_(D)≧1 in this case. Each data symbol streammay be steered to one, multiple, or all receive antennas. For a MISOsystem with one receive antenna, one steering vector is obtained for thesingle receive antenna and used to transmit one data symbol stream.

For all of the eigensteering schemes, a matched filter is derived foreach receive antenna based on the steering vector(s) used by thetransmitter and the sequence of channel (impulse or frequency) responsevectors for the receive antenna. The received symbol stream for eachreceive antenna is filtered with the matched filter for that receiveantenna to obtain one or more filtered symbol substreams. The filteredsymbol substreams from all NR matched filters for the NR receiveantennas are then combined to obtain N_(D) detected symbol streams forthe N_(D) data streams sent by the transmitter, where N_(D)≧1 in thiscase. Equalization and other post-processing may be performed on theN_(D) detected symbol streams to obtain N_(D) recovered symbol streams,which are estimates of the N_(D) data symbol streams sent by thetransmitter.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 shows a transmitter and a receiver in a MISO system;

FIG. 2 shows a transmit (TX) data processor in the MISO system;

FIGS. 3A, 3B, and 3C show three embodiments of a TX spatial processor inthe MISO system;

FIGS. 4A, 4B, and 4C show three embodiments of a receive (RX) spatialprocessor in the MISO system;

FIG. 5 shows a block diagram of the receiver in the MISO system;

FIG. 6 shows a transmitter and a receiver in a MIMO system;

FIG. 7 shows a TX data processor in the MIMO system;

FIGS. 8A, 8B, and 8C show three embodiments of a TX spatial processor inthe MIMO system;

FIGS. 9A through 9F show six embodiments of an RX spatial processor inthe MIMO system;

FIG. 10 shows a block diagram of the receiver in the MIMO system;

FIG. 11 shows a process for performing principal mode eigensteering,multi-mode eigensteering, and main path eigensteering in the MIMOsystem; and

FIG. 12 shows a process for performing receiver eigensteering in theMISO or MIMO system.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

The eigensteering techniques described herein may be used for variouswireless communication systems including single-carrier andmulti-carrier MISO and MIMO systems. Multiple carriers may be providedby OFDM or some other multiple-carrier modulation technique orconstruct. In the description below, the term “MIMO system” genericallyrefers to both single-carrier and multi-carrier MIMO systems.

For clarity, the following notations are used for the description below.Time-domain variables are functions of n and are denoted with cursivetexts (e.g., h(n)). Frequency-domain variables are functions of k andare denoted with plain texts (e.g., h(k)). Vectors are denoted withlower-case bold-faced and underlined texts (e.g., h(n) and h(k)).Matrices are denoted with upper-case bold-faced and underlined texts(e.g., H((n) and H(k)). Three-dimension matrices are denoted withupper-case bold-faced and doubly underlined texts (e.g., H and H).

i) MISO System

A time-dispersive MISO channel with N_(T) transmit antennas and a singlereceive antenna may be characterized by a time-domain channel impulseresponse matrix H with dimensions of (L+1)×N_(T), where L denotes thedelay extent of the MISO channel in symbol periods. The delay extent ofa channel is the difference between the earliest and latest resolvablepropagation paths in the channel. The matrix H is composed of N_(T)channel impulse response vectors h _(j), for j=1, 2, . . . N_(T), orequivalently, L+1 row vectors h(n), for n=0, 1, . . . L, which may beexpressed as: ℋ _ = [ ⁢ ( 0 ) ⁢ ( 1 ) ⋮ ⁢ ( L ) ] ⁢ = [ ⁢ ( 0 ) ⁢ ( 0 ) ⋯ ⁢ ( 0) ⁢ ( 1 ) ⁢ ( 1 ) ⋯ ⁢ ( 1 ) ⋮ ⋮ ⋰ ⋮ ⁢ ( L ) ⁢ ( L ) ⋯ ⁢ ( L ) ] = [ 1 ⁢ 2 ⁢   ⁢ …⁢  ⁢ N T ] , Eq ⁢   ⁢ ( 1 )where entry h_(j)(n), for j=1, 2, . . . N_(T) and n=0, 1, . . . L, isthe coupling (i.e., complex gain) between transmit antenna j and thereceive antenna for time delay n. Each vector h _(j), for j=1, 2, . . .N_(T), includes L+1 complex values for the channel impulse responsebetween transmit antenna j and the receive antenna. Each row vectorh(n), for n=0, 1, . . . L, includes N_(T) complex values for the channelgains between the N_(T) transmit antennas and the receive antenna fortime delay n.

The channel impulse response may be estimated by the receiver based onpilot symbols sent by the transmitter. The transmitter can “cover” thepilot for each transmit antenna with a unique orthogonal sequenceassigned to that antenna. Covering is a process whereby a givenmodulation symbol p (or a set of W symbols having the same value) to betransmitted is multiplied by all W chips of a W-chip orthogonal sequenceto obtain W covered symbols, which are then transmitted. N_(T)orthogonal pilots can be obtained with N_(T) orthogonal sequences forthe N_(T) transmit antennas. The covering achieves orthogonality amongthe N_(T) pilots sent from the N_(T) transmit antennas and allows thereceiver to distinguish the individual transmit antennas.

The receiver can “decover” the received pilot symbols with each of thesame N_(T) orthogonal sequences to estimate the channel impulse responsebetween each of the N_(T) transmit antennas and the receive antenna.Decovering is a complementary process whereby W received symbols for Wcovered symbols are multiplied by the W chips of the same W-chiporthogonal sequence to obtain W decovered symbols, which are thenaccumulated to obtain an estimate of the transmitted symbol p. Thedecovering is performed at L+1 time delays to obtain the L+1 row vectorsh(n), for n=0, 1, . . . L, for the channel impulse response of the MISOchannel.

A time dispersive MISO channel may also be characterized by atwo-dimensional frequency-domain channel frequency response matrix Hwith dimensions of N_(F)×N_(T), where N_(F) is the number of frequencysubbands and N_(F)≧(L+1). The matrix H is composed of N_(T) channelfrequency response vectors h _(j), for j=1, 2, . . . N_(T), i.e., H=[h ₁h ₂ . . . h _(N) _(T) ]. Each vector h _(j) includes N_(F)frequency-domain values that may be obtained by performing anN_(F)-point discrete Fourier transform (DFT) on the L+1 time-domainvalues of the corresponding vector h _(j) of the matrix H. For eachtransmit-receive antenna pair, there is thus a one-to-one correspondencebetween the time-domain channel impulse response vector h _(j) and thefrequency-domain channel frequency response vector h _(j). The matrix His equivalently composed of N_(F) row vectors h(k), for k=1, 2, . . .N_(F), i.e., H=[h ^(T) (1) h ^(T) (₂) . . . h ^(T) (N_(F))]^(T), whereM^(T) is the transpose of M. Each row vector h(k) includes N_(T) complexgain values for the frequency response between the N_(T) transmitantennas and the receive antenna for subband k. The DFT to obtain h(k)may be expressed as: $\begin{matrix}{{{\underset{\_}{h}(k)} = {\sum\limits_{n = 0}^{L}{(n) \cdot {\mathbb{e}}^{{- {j{({2{\pi/N_{F}}})}}}{({k - 1})}n}}}},{{{for}k} = 1},2,{\ldots\quad{N_{F}.}}} & {{Eq}\quad(2)}\end{matrix}$

The transmitter may perform eigensteering on a data symbol stream s(n)prior to transmission via the MISO channel in order to achieve higherreceived signal-to-noise ratio (SNR) for the data symbol stream. Theeigensteering may be performed using a frequency-independent steeringvector v_(miso) which may be derived to maximize the received SNR orbased on some other criteria.

In one embodiment, the steering vector v_(miso) for the transmitter isobtained by first computing an N_(T)×N_(T) correlation matrix R_(miso)as follows: R _ miso = ∑ n = 0 L ⁢ H ⁢ ( n ) ⁢ ⁢ ( n ) = 1 N F ⁢ ∑ k = 1 N F ⁢h _ H ⁡ ( k ) ⁢ h _ ⁡ ( k ) , Eq ⁢   ⁢ ( 3 )where h ^(H) is the conjugate transpose of h. The matrix R_(miso) may beviewed as an average of either L+1 individual correlation matrices ofh(n) for L+1 time delays or N_(F) individual correlation matrices ofh(k) for N_(F) subbands. The individual correlation matrices are givenequal weight in equation (3). In another embodiment, the individualcorrelation matrices may be given unequal weights in the computation ofR_(miso). For example, each individual correlation matrix may beweighted by the energy associated with that matrix, which can becomputed as described below.

Eigenvalue decomposition of the correlation matrix R_(miso) is thenperformed as follows:R _(miso) =V _(miso)Λ_(miso) V _(miso) ^(H)   Eq (4)where V_(miso) is an N_(T)×N_(T) unitary matrix whose columns are theeigenvectors of R_(miso); and

Λ_(miso) is an N_(T)×N_(T) diagonal matrix whose diagonal entries arethe eigenvalues of R_(miso).A unitary matrix M is characterized by the property M^(H)M=I, where I isthe identity matrix with ones along the diagonal and zeros elsewhere.The N_(T) eigenvectors of the unitary matrix V_(miso), denoted as v_(j)for j=1, 2, . . . N_(T), are thus orthogonal to one another. Moreover,the length of each eigenvector is equal to one, i.e.,${{{\underset{\_}{v}}_{j}}^{2} = {{\sum\limits_{i = 1}^{N_{T}}{\nu_{i,j}}^{2}} = 1}},$where v_(j)=[v_(1,j) v_(2,j) . . . v_(N) _(T) _(,j)]^(T). The N_(T)eigenvectors are also referred to as steering vectors and may be usedfor eigensteering by the transmitter and matched filtering by thereceiver, as described below.

The matrix R_(miso) is of dimension N_(T)×N_(T) and rank N_(miso), whereN_(miso)≦min {N_(T), (L+1)}. The diagonal matrix Λ_(miso) thus containsN_(miso) positive real values along the diagonal and zeros elsewhere.The largest non-zero entry is referred to as the principal eigenvalueλ_(miso) of the matrix R_(miso) and is indicative of the power gain forthe spatial channel (or “time-domain eigenrode”) corresponding to thateigenvalue. The frequency-independent steering vector v_(miso) to usefor eigensteering is the “principal” eigenvector of R_(miso), which isthe column of V_(miso) that corresponds to the principal eigenvalue ofR_(miso).

The transmitter performs eigensteering on the data symbol stream s(n)with the steering vector v_(miso) to obtain N_(T) transmit symbolstreams χ_(miso)(n), as follows:χ_(miso)(n)=s(n)·v _(miso)   Eq (5)With the eigensteering shown in equation (5), the data symbol streams(n) observes an effective channel that is a single-input single-output(SISO) channel with an effective channel impulse response ofh(n)v_(miso), for n 32 0, 1, . . . L. The N_(T) transmit symbol streamsχ_(miso)(n) are further processed and transmitted from the N_(T)transmit antennas to the receiver.

The receiver obtains a received symbol stream y_(miso)(n) from thesingle receive antenna, which may be expressed as: $\begin{matrix}\begin{matrix}{{{{\mathcal{y}}_{miso}(n)} = {{(n) \otimes {{\underset{\_}{\mathcal{x}}}_{miso}(n)}} + {n_{miso}(n)}}},} \\{{= {\left( {\sum\limits_{j = 0}^{L}{(j){s\left( {n - j} \right)}{\underset{\_}{v}}_{miso}}} \right) + {n_{miso}(n)}}},}\end{matrix} & {{Eq}\quad(6)}\end{matrix}$where “{circle around (×)}” denotes a convolution and n_(miso)(n) isadditive white Gaussian noise (AWGN). The received symbol streamy_(miso)(n) experiences intersymbol interference (ISI), which is aphenomenon whereby each symbol in a received stream acts as distortionto subsequent symbols in the received stream. Intersymbol interferencemay be mitigated through the use of OFDM in conjunction with asufficiently long cyclic prefix, as described below. Alternatively, fora single-carrier MISO system, intersymbol interference may be mitigatedthrough the use of appropriate temporal matched filtering in combinationwith equalization, as also described below.

The receiver can perform matched filtering of the received symbol streamy_(miso)(n) in either the time domain or the frequency domain. Thetime-domain matched filtering may be expressed as: s ~ miso ⁡ ( n ) = v _miso H ⁢ H ⁢ ( L - n ) ⊗ 𝓎 miso ⁡ ( n ) , = v _ miso H ⁢ ∑ j = 0 L ⁢ H ⁢ ( L -j ) · 𝓎 miso ⁡ ( n - j ) , Eq ⁢   ⁢ ( 7 )where {tilde over (s)}_(miso)(n) denotes a detected symbol stream, whichis an estimate of the data symbol stream s(n) sent by the transmitter.The matched filter m_(miso)(n)=v_(miso) _(H) h ^(H)(L−n), for n=0, 1, .. . L, maximizes the received SNR.

An equalizer may be used to mitigate intersymbol interference due totime dispersion in the MIMO channel. The equalizer may be a minimum meansquare error (MMSE) equalizer, a decision feedback equalizer (DFE), amaximum likelihood sequence estimator (MLSE), or some other type ofequalizer. The equalizer may be implemented with an adaptive filterhaving coefficients that can be updated with pilot and/or data symbolsand based on a particular criterion (e.g., minimum mean square error).The equalizer performs equalization on the detected symbol stream {tildeover (s)}_(miso)(n) and provides a recovered symbol stream ŝ_(miso)(n),which is a better estimate of the data symbol stream s(n) sent by thetransmitter. In general, the detected symbol stream {tilde over(s)}_(miso)(n) may be provided directly as the recovered symbol streamŝ_(miso) (n) or may be post-processed (e.g., equalized) to obtain therecovered symbol stream ŝ_(miso) (n).

The frequency-domain matched filtering may be expressed as:ŝ _(miso)(k)=v _(miso) ^(H) h ^(H)(k)y _(miso)(k), for k=1, 2, . . .N_(F),   Eq (8)where ŝ_(miso)(k) is the recovered symbol substream for subband k; and

y_(miso)(k) is the received symbol substream for subband k.

The N_(F) received symbol substreams y_(miso)(k), for k=1, 2, . . .N_(F), may be obtained by performing a fast Fourier transform (FFT) ofeach set of N_(F) symbols in the received symbol stream y_(miso)(n). Thematched filter m_(miso)(k)=v_(miso) ^(H) h ^(H)(k), for k=1, 2, . . .N_(F), is a complex-valued scalar that maximizes the received SNR foreach subband. The N_(F) recovered symbol substreams for the N_(F)subbands may be multiplexed together to obtain the recovered symbolstream ŝ_(miso)(n).

For both time-domain and frequency-domain matched filtering, thereceived SNR may be expressed as: $\begin{matrix}\begin{matrix}{{SNR}_{miso} = {\frac{P_{total}}{\sigma^{2}}{\underset{\_}{v}}_{miso}^{H}{\underset{\_}{R}}_{miso}{\underset{\_}{v}}_{miso}}} \\{= {\frac{P_{total}}{\sigma^{2}}\lambda_{{miso},}}}\end{matrix} & {{Eq}\quad(9)}\end{matrix}$where P_(total) is the total transmit power used by the transmitter forthe data symbol stream;

σ² is the noise floor at the receiver; and

λ_(miso) is the principal eigenvalue of R_(miso).

The capacity C_(miso) ^(fi) of the MISO channel withfrequency-independent eigensteering may be determined usingfrequency-domain analysis and assuming that the same steering vector isused for all N_(F) subbands. The capacity C_(miso) ^(fi) may beexpressed as: $\begin{matrix}\begin{matrix}{C_{miso}^{fi} = {\sum\limits_{k = 1}^{N_{F}}{\log_{2}\left( {1 + {{SNR}(k)}} \right)}}} \\{{= {\sum\limits_{k = 1}^{N_{F}}{\log_{2}\left( {1 + {{\rho \cdot {\underset{\_}{v}}_{miso}^{H}}{\underset{\_}{R}(k)}{\underset{\_}{v}}_{miso}}} \right)}}},}\end{matrix} & {{Eq}\quad(10)}\end{matrix}$where ρ is the average received SNR as measured at the receive antenna,which is equal to the total receive power divided by the receiver noiseσ². The matrix R(k) is the correlation matrix of h(k), which may beobtained and decomposed as follows:R(k)= h ^(H)(k) h (k)=U(k)Λ(k)U ^(H)(k), for k=1, 2, . . . N_(F),   Eq (11)where Λ(k) is the diagonal matrix of eigenvalues of R(k), and

U(k) is the unitary matrix of eigenvectors of R(k).

The quadratic term in equation (10) may be expressed as:v _(miso) ^(H) R(k)v _(miso) =v _(miso) ^(H) U(k)Λ(k)U ^(H)(k)v _(miso)=z ^(H)(k)Λ(k)z(k),   Eq (12)where z(k)=U^(H)(k)v_(miso). Since R(k) has only one non-zeroeigenvalue, equation (12) may be simplified as follows:v _(miso) ^(H) R(k)v _(miso) =|z ₁(k)|²λ(k),   Eq (13)where λ(k) is the non-zero eigenvalue of R(k), which is λ(k)=∥h(k)∥² fora MISO channel, and z₁(k) is the element of z(k) corresponding to theeigenvalue λ(k). The capacity C_(miso) ^(fi) of the MISO channel withfrequency-independent eigensteering may then be expressed as:$\begin{matrix}{C_{miso}^{fi} = {\sum\limits_{k = 1}^{N_{F}}{{\log_{2}\left( {1 + {\rho \cdot {{z_{i}(k)}}^{2} \cdot {\lambda(k)}}} \right)}.}}} & {{Eq}\quad(14)}\end{matrix}$

The capacity C_(miso) ^(n) for the MISO channel without eigensteering atthe transmitter (or equivalently with a steering vector v=[g g . . . g],where g=√{square root over (1/N _(T) )}) may be expressed as:$\begin{matrix}{C_{miso}^{n} = {\sum\limits_{k = 1}^{N_{F}}{{\log_{2}\left( {1 + {\frac{\rho}{N_{T}} \cdot {\lambda(k)}}} \right)}.}}} & {{Eq}\quad(15)}\end{matrix}$In general, the capacity C_(miso) ^(fi) of the MISO channel withfrequency-independent eigensteering is greater than the capacityC_(miso) ^(n) of the MISO channel without eigensteering.

An exemplary method has been described above for obtaining thefrequency-independent steering vector v_(miso) for eigensteering at thetransmitter in the MISO system. The steering vector may also be obtainedin other manners, and this is within the scope of the invention.

The frequency-independent eigensteering may also be used for a MISOsystem that employs OFDM (i.e., a MISO-OFDM system). The transmitter canperform eigensteering in the time-domain, as shown in equation (5),where s(n) denotes a sequence of time-domain chips for OFDM symbolsgenerated for the data stream by OFDM modulation. OFDM modulation isdescribed below. The transmitter can also perform eigensteering in thefrequency-domain on the data symbols for each subband, prior to the OFDMmodulation to generate OFDM symbols. The receiver can perform matchedfiltering in the time-domain, as shown in equation (7), or thefrequency-domain, as shown in equation (8).

ii) MIMO System

A time-dispersive MIMO channel with N_(T) transmit antennas and N_(R)receive antennas may be characterized by a three-dimensional time-domainchannel impulse response matrix H with dimensions of N_(R)×N_(T)×(L+1).The matrix H is composed of L+1 channel impulse response matrices H(n),for n=0, 1, . . . L, i.e., H=[H(0) H(1) . . . H(L)], which may beexpressed as: ℋ _ ⁡ ( n ) = [ 1 ⁢ ( n ) 2 ⁢ ( n ) ⋮ N R ⁢ ( n ) ] = [ 1 , 1 ⁢( n ) 1 , 2 ⁢ ( n ) ⋯ 1 , N T 2 , 1 ⁢ ( n ) 2 , 2 ⁢ ( n ) ⋯ 2 , N T ⁢ ( n )⋮ ⋮ ⋰ ⋮ N R , 1 ⁢ ( n ) N R , 2 ⁢ ( n ) ⋯ N R , N T ⁢ ( n ) ] , Eq ⁢   ⁢ ( 16)for n=0, 1, . . . L, where entry h_(i,j)(n), for i=1, 2, . . . N_(R),j=1, 2, . . . N_(T), and n=0, 1, . . . L, is the coupling (i.e., complexgain) between transmit antenna j and receive antenna i for time delay n.The row vector h _(i)(n), for i=1, 2, . . . N_(R) and n=0, 1, . . . L,includes N_(T) complex values for the channel gains between the N_(T)transmit antennas and receive antenna i for time delay n.

The channel impulse response may be estimated by the receiver based onpilot symbols sent by the transmitter. In one embodiment, thetransmitter covers the pilot for each transmit antenna with anorthogonal code assigned to that antenna. The pilots sent from the N_(T)transmit antennas are covered by N_(T) orthogonal codes and may beindividually recovered. At the receiver, the received pilot from eachreceive antenna i is decovered with the N_(T) orthogonal codes at aparticular time delay to obtain the channel response between receiveantenna i and each of the N_(T) transmit antennas for that time delay,i.e., one row of the matrix H(n). The decovering is performed separatelyfor all N_(R) receive antennas to obtain the N_(R) rows of the matrixH(n). The decovering is also performed at L+1 time delays (i.e., forn=0, 1, . . . L) for each transmit-receive antenna pair to obtain theL+1 time-domain values for the channel impulse response for thattransmit-receive antenna pair.

A time dispersive MIMO channel may also be characterized by acorresponding three-dimensional frequency-domain channel frequencyresponse matrix H with dimensions of N_(R)×N_(T)×N_(F), where N_(F)>L.The matrix H is composed of N_(F) channel frequency response matricesH(k), for k=1, 2, . . . N_(F), which can be obtained by computing anN_(F)-point discrete Fourier transform on the L+1 channel impulseresponse matrices H(n), for n=0, 1, . . . L, as follows: $\begin{matrix}{{{\underset{\_}{H}(k)} = {\begin{bmatrix}{{\underset{\_}{h}}_{1}(k)} \\{{\underset{\_}{h}}_{2}(k)} \\\vdots \\{{\underset{\_}{h}}_{N_{R}}(k)}\end{bmatrix} = {\sum\limits_{n = 0}^{L}{{\underset{\_}{\mathcal{H}}(n)} \cdot {\mathbb{e}}^{{- {j{({2{\pi/N_{F}}})}}}{({k - 1})}n}}}}},{{{for}\quad k} = 1},2,{\ldots\quad{N_{F}.}}} & {{Eq}\quad(17)}\end{matrix}$Each matrix H(k), for k=1, 2, . . . N_(F), includes N_(R) row vectors h_(i)(k), for i=1, 2, . . . N_(R), for the N_(R) receive antennas. Eachrow vector h _(i)(k) includes N_(T) complex values for the channel gainsbetween the N_(T) transmit antennas and receive antenna i for subband k.Each entry h _(i,j) of the matrix H, for i=1, 2, . . . N_(R) and j=1, 2,. . . N_(T), includes N_(F) frequency-domain values that may be obtainedby taking the DFT of the L+1 time-domain values for a correspondingentry h _(i,j) of the matrix H. For each transmit-receive antenna pair,there is thus a one-to-one correspondence between the channel impulseresponse h _(i,j) and the channel frequency response h _(i,j).

Frequency-independent eigensteering may be performed in various mannersfor a MIMO system. Some exemplary frequency-independent eigensteeringschemes are described below.

A. Principal-Mode Eigensteering

For principal-mode eigensteering, a data symbol stream is transmitted onthe principal spatial channel of the MIMO channel using a singlefrequency-independent steering vector v_(pm). To obtain this steeringvector, an N_(T)×N_(T) correlation matrix R_(mimo) is first computed asfollows: R _ mimo = ∑ n = 0 L ⁢ ℋ _ H ⁡ ( n ) ⁢ ℋ _ ⁡ ( n ) = 1 N F ⁢ ∑ k = 1N F ⁢ H _ H ⁡ ( k ) ⁢ H _ ⁡ ( k ) , = ∑ n = 0 L ⁢ ∑ j = 1 N T ⁢ j H ⁢ ( n ) ⁢ j ⁢( n ) = 1 N F ⁢ ∑ k = 1 N F ⁢ ∑ j = 1 N T ⁢   ⁢ h _ H ⁡ ( k ) ⁢ h _ ⁡ ( k ) .Eq ⁢   ⁢ ( 18 )

Eigenvalue decomposition of R_(mimo) is then performed as follows:R_(mimo)=V_(mimo)Λ_(mimo)V_(mimo) ^(H),   Eq (19)where V_(mimo) is a unitary matrix of eigenvectors of R_(mimo) andΛ_(mimo) is a diagonal matrix whose diagonal entries are the eigenvaluesof R_(mimo).

A MIMO channel may be decomposed into N_(S) spatial channels, whereN_(S)≦min {N_(T), N_(R)}. The matrix R_(mimo) is of rank N_(S) and thediagonal matrix Λ_(mimo) contains N_(S) non-negative real values alongthe diagonal. The largest non-zero diagonal entry is referred to as theprincipal eigenvalue λ_(mimo) of the matrix R_(mimo). In an embodiment,the steering vector v_(pm) to use for eigensteering is the principaleigenvector of R_(mimo), which is the column of V_(mimo) thatcorresponds to the principal eigenvalue of R_(mimo). The steering vectorv_(pm) may be viewed as being for the principal spatial channel of the“averaged” MIMO channel.

The transmitter performs eigensteering on the data symbol stream s(n)with the steering vector v_(pm) to obtain N_(T) transmit symbol streamsχ_(pm)(n), as follows:χ_(pm)(n)=s(n)·v _(pm)   Eq (20)With the eigensteering shown in equation (20), the data symbol streams(n) observes an effective channel that is a single-inputmultiple-output (SIMO) channel with an effective channel impulseresponse of H(n)v_(pm), for n=0, 1, . . . L. The N_(T) transmit symbolstreams χ_(pm) (n) are further processed and sent from the N_(T)transmit antennas to the receiver.

The receiver obtains N_(R) received symbol streams y _(pm)(n) from theN_(R) receive antennas, which may be expressed as:y _(pm)(n)=H(n){circle around (×)}χ_(pm)(n)+n _(mimo)(n)   Eq (21)where n_(mimo)(n) is additive white Gaussian noise with a mean vector of0 and a covariance matrix of Λ_(n)=σ²I, where 0 is a vector of allzeros. The receiver can perform matched filtering of the received symbolstreams y _(pm)(n) in either the time domain or the frequency domain.

The time-domain matched filtering may be expressed as:{tilde over (s)} _(pm)(n)=v _(pm) ^(H)(L−n){circle around (×)} y_(pm)(n), for n=0, 1, . . . L.   Eq (22)The receiver matched filter is m_(pm)(n)=v_(pm) ^(H)H^(H)(L−n), for n=0,1 . . . L, which includes N_(R) individual matched filters for the N_(R)receive antennas. The matched filter m_(pm,i)(n) for each receiveantenna has an impulse response of m_(pm,i)(n)=v_(pm) ^(H) h _(i)^(H)(L−n), for n=0, 1, . . . L, that maximizes the received SNR for thatreceive antenna. The outputs of the N_(R) individual matched filters forthe N_(R) receive antennas are summed to obtain the detected symbolstream {tilde over (s)}_(pm)(n). Post processing (e.g., equalization)may be performed on the detected symbol stream _(pm)(n) to obtain therecovered symbol stream ŝ_(pm)(n).

The frequency-domain matched filtering may be expressed as:ŝ _(pm)(k)=v _(pm) ^(H) H ^(H)(k) y _(pm)(k), for k=1, 2, . . . N_(F),  Eq(23)where y _(pm)(k) is the received symbol substreams for subband k, whichcan be obtained by performing an FFT of each set of N_(F) symbols in thereceived symbol streams y _(pm) (n). The receiver matched filter is m_(pm)(k)=v_(pm) ^(H)H^(H)(k), for k=1, 2 . . . N_(F), which includesN_(R) individual matched filters for the N_(R) receive antennas. Thematched filter m_(pm,i)(k) for each receive antenna i has a response ofm_(pm,i)(k)=v_(pm) ^(H) h _(i) ^(H)(k), for k=1, 2, . . . N_(F). Theoutputs of the N_(R) individual matched filters for the N_(R) receiveantennas for each subband k are summed to obtain the recovered symbolsubstream ŝ_(pm)(k) for that subband. The N_(F) recovered symbolsubstreams for the N_(F) subbands may be multiplexed to obtain therecovered symbol stream ŝ_(pm)(n).

For both the time-domain and frequency-domain matched filtering, thereceived SNR, averaged across the N_(R) receive antennas, may beexpressed as: $\begin{matrix}{{SNR}_{mimo}^{pm} = {\frac{\rho}{N_{R}}{\underset{\_}{v}}_{pm}^{H}{\underset{\_}{R}}_{mimo}{{\underset{\_}{v}}_{pm}.}}} & {{Eq}\quad(24)}\end{matrix}$The matched filter V_(pm) ^(H)H^(H)(L−n), for n=0, 1, . . . L, maximizesthe received SNR.

The capacity C_(mimo) ^(pm) of the MIMO channel with principal-modeeigensteering may be expressed as: $\begin{matrix}{C_{mimo}^{pm} = {\sum\limits_{k = 1}^{N_{F}}{{\log_{2}\left( {1 + {{\rho \cdot {\underset{\_}{v}}_{pm}^{H}}{{\underset{\_}{H}}^{H}(k)}{\underset{\_}{H}(k)}{\underset{\_}{v}}_{pm}}} \right)}.}}} & {{Eq}\quad(25)}\end{matrix}$The quadratic term in equation (25) may be expressed as: $\begin{matrix}{{{{\underset{\_}{v}}_{pm}^{H}{{\underset{\_}{H}}^{H}(k)}{\underset{\_}{H}(k)}{\underset{\_}{v}}_{pm}} = {\sum\limits_{\ell = 1}^{N_{T}}{{{{\underset{\_}{v}}_{pm}^{H}{{\underset{\_}{u}}_{\ell}(k)}}}^{2} \cdot {\lambda_{\ell}(k)}}}},} & {{Eq}\quad(26)}\end{matrix}$where u_(l)(k) is the eigenvector associated with the l-th eigenvalueλ_(l)(k) of the correlation matrix R(k)=H^(H)(k)H(k). The capacityCm_(mimo) ^(pm) may then be expressed as: $\begin{matrix}{C_{mimo}^{pm} = {\sum\limits_{k = 1}^{N_{F}}{{\log_{2}\left( {1 + {\rho \cdot {\sum\limits_{\ell = 1}^{N_{T}}{{{{\underset{\_}{v}}_{pm}^{H}{{\underset{\_}{u}}_{\ell}(k)}}}^{2} \cdot {\lambda_{\ell}(k)}}}}} \right)}.}}} & {{Eq}\quad(27)}\end{matrix}$

B. Multi-Mode Eisensteering

For multi-mode eigensteering, multiple data symbol streams aretransmitted on multiple spatial channels of the MIMO channel usingmultiple frequency-independent steering vectors in a matrix V_(mm),whose columns are the eigenvectors of the correlation matrix R_(mimo).Since R_(mimo) is of rank N_(s), where N_(S)≦min {N_(T), N_(R)}, thematrix V_(mm) may include up to N_(S) eigenvectors v_(l), for l=1, 2, .. . N_(S), for up to N_(S) eigenmodes of R_(mimo). For clarity, thefollowing description assumes that all N_(S) eigenmodes are used fordata transmission.

The transmitter performs eigensteering on N_(S) data symbol streamss_(mm)(n) with the steering matrix V_(mm) to obtain N_(T) transmitsymbol streams χ_(mm)(n), as follows:χ_(m)(n)=V _(mm) s (n),   Eq (28)where s _(mm)(n)=[s₁(n) s₂(n) . . . s_(N) _(S) (n)]^(T), V_(mm)=[v₁ v₂ .. . v_(N) _(S) ], and N_(S)≦min{N_(T), N_(R)} for a full-rank MIMOchannel. Each data symbol stream s_(l)(n), for l=1, 2, . . . N_(S), issteered with a respective steering vector v_(l) in the matrix V_(mm).Each data symbol stream s_(l)(n) observes an effective channel that is aSIMO channel with an effective channel impulse response of H(n)v_(l),for n=0, 1, . . . L. The N_(T) transmit symbol streams χ_(mm)(n) arefurther processed and sent from the N_(T) transmit antennas to thereceiver.

The receiver obtains N_(R) received symbol streams y _(mm)(n) from theN_(R) receive antennas, which y _(mm)(n)=H(n){circle around(×)}χ_(mm)(n)+n _(mimo)(n). For a time dispersive MIMO channel, theeigensteering with multiple frequency-independent steering vectors inthe matrix V_(mm) does not diagonalize the channel. Thus, when multiplespatial channels are used for data transmission usingfrequency-independent eigensteering, there will in general be cross-talkbetween the multiple symbol streams as well as intersymbol interferenceat the receiver.

The receiver can perform matched filtering of the received symbolstreams y _(mm)(n) in either the time domain or the frequency domain.The time-domain matched filtering may be expressed as:{tilde over (s)} _(mm)(n)=V _(mm) ^(H) H ^(H)(L−n){circle around (×)} y_(mm)(n),   Eq (29)where {tilde over (s)}_(mm)(n) denotes N_(S) detected symbol streams.The receiver matched filter is M_(mm)(n)=V_(mm) ^(H)h^(H)(L−n), for n=0,1, . . . L, which includes N_(R) individual matched filters for theN_(R) receive antennas. The matched filter m _(mm,i)(n) for each receiveantenna has an impulse response of m _(mm,i)(n)=V_(mm) ^(H) h _(i)^(H)(L−n), for n=0, 1, . . . L. The output of the matched filter foreach receive antenna includes N_(S) filtered symbol substreamscorresponding to the N_(S) steering vectors (i.e., N_(S) columns ofV_(mm)). The N_(R) filtered symbol substreams from the N_(R) matchedfilters for each steering vector are combined to obtain the detectedsymbol stream {tilde over (s)}_(l)(n) for that steering vector. N_(S)detected symbol streams {tilde over (s)}_(mm)(n) are obtained for theN_(S) data symbols s _(mm)(n) sent by the transmitter.

The frequency-domain matched filtering may be expressed as:{tilde over (s)} _(mm)(k)=V _(mm) ^(H) H ^(H)(k) y _(mm)(k), for k=1, 2,. . . N_(F),   Eq (30)where y _(mm)(k) is the received symbol substream for subband k, whichis obtained by performing an FFT of each set of N_(F) symbols in thereceived symbol stream y _(mm)(n). The receiver matched filter isM_(mm)(k)=V_(mm) ^(H)H^(H)(k), for k=1, 2, . . . N_(F), which includesN_(R) individual matched filters for the N_(R) receive antennas. Thematched filter m _(mm,i)(k) for each receive antenna has a response of m_(mm,i)(k)=V_(mm) ^(H) h _(i) ^(H)(k), for k=1, 2, . . . N_(F). For eachsubband k, the output of the matched filter for each receive antennaincludes N_(S) filtered symbol substreams corresponding to the N_(S)transmit steering vectors. For each subband k, the N_(R) filtered symbolsubstreams from the N_(R) matched filters for each steering vector arecombined to obtain the detected symbol substream {tilde over (s)}_(l)(k)for that steering vector. The N_(F) detected symbol substreams for theN_(F) subbands for each steering vector are then multiplexed to obtainthe detected symbol stream {tilde over (s)}_(l)(n) for the data symbolstream s_(l)(n) sent with that steering vector. N_(S) detected symbolstreams {tilde over (s)}_(mm)(n) are obtained for N_(S) data symbolstreams s _(mm)(n) sent by the transmitter.

As noted above, if multiple data symbol streams are transmittedsimultaneously, then there is cross-talk between these data symbolstreams at the receiver for a time dispersive MIMO channel. A space-timeor “joint” equalizer may be used to mitigate the cross-talk andintersymbol interference due to time dispersion in the MIMO channel. Thespace-time equalizer may be a minimum mean square error linear equalizer(MMSE-LE), a decision feedback equalizer (DFE), a maximum likelihoodsequence estimator (MLSE), or some other type of equalizer. Thespace-time equalizer may be designed to operate on the N_(S) detectedsymbol streams {tilde over (s)}_(mm)(n) in both time and space domainsto obtain N_(S) recovered symbol streams ŝ_(mm)(n), which are improvedestimates of the data symbol streams s _(mm)(n) sent by the transmitter.Exemplary designs of the MMSE-LE, DFE, and MLSE are described incommonly assigned U.S. patent application Ser. No. 09/993,087, entitled“Multiple-Access Multiple-Input Multiple-Output (MIMO) CommunicationSystem,” filed Nov. 6, 2001.

The space-time equalizer may also implement a successive equalizationand interference cancellation receiver processing technique, whichsuccessively recovers one data symbol stream at a time. As each datasymbol stream is recovered, the interference it causes to the remaining,not yet recovered data symbol streams is estimated and canceled from thedetected symbol streams to obtain “modified” symbol streams. Themodified symbol streams are then processed to recover the next datasymbol stream. The process is repeated until all N_(S) data symbolstreams are recovered. By removing the interference due to eachrecovered data symbol stream, the not yet recovered data symbol streamsexperience less interference and may achieve higher SNRs. The successiveequalization and interference cancellation receiver processing techniqueis also described in the aforementioned U.S. patent application Ser. No.09/993,087.

The capacity C_(mimo) ^(mm) of the MIMO channel with multi-modeeigensteering may be expressed as: $\begin{matrix}{{C_{mimo}^{pm} = {\sum\limits_{k = 1}^{N_{F}}{\log_{2}{{\underset{\_}{I} + {{\frac{\rho}{N_{S}} \cdot {\underset{\_}{V}}_{m\quad m}^{H}}{\underset{\_}{R}(k)}{\underset{\_}{V}}_{m\quad m}}}}}}},} & {{Eq}\quad(31)}\end{matrix}$where |M| denotes the determinant of M. A correlation matrix {tilde over(R)}(k) may be defined as {tilde over (R)}(k)=V_(mm) ^(H)R(k)V_(mm). Theeigenvalues of {tilde over (R)}(k) may be computed and denoted as {tildeover (λ)}_(l)(k), for l=1, 2, . . . N_(S) and k=1, 2, . . . N_(F). Thecapacity C_(mimo) ^(mm) of the MIMO channel with multi-modeeigensteering may then be expressed as: $\begin{matrix}{C_{mimo}^{m\quad m} = {\sum\limits_{k = 1}^{N_{F}}{\sum\limits_{\ell = 1}^{N_{S}}{{\log_{2}\left( {1 + {\frac{\rho}{N_{S}} \cdot {{\overset{\sim}{\lambda}}_{\ell}(k)}}} \right)}.}}}} & {{Eq}\quad(32)}\end{matrix}$

C. Main Path Eigensteering

For main path eigensteering, a data symbol stream is transmitted on theprincipal spatial channel for the main propagation path of the MIMOchannel using a single frequency-independent steering vector v_(mp), Asnoted above, a time dispersive MIMO channel can be characterized by L+1channel impulse response matrices H(n), for n=0, 1, . . . L. In anembodiment, the main path is defined as the propagation path with themost received energy. The energy E(n) of each channel impulse responsematrix H(n), for n=0, 1, . . . L, may be computed as follows: E ⁡ ( n ) = ℋ _ ⁡ ( n )  2 = ∑ j = 1 N R ⁢ ∑ i = 1 N T ⁢  i , j ⁢ ( n )  2 , ⁢ for ⁢  ⁢ n = 0 , 1 , … ⁢   ⁢ L . Eq ⁢   ⁢ ( 33 )The energy E(n) is also (1) the trace of the correlation matrixR(n)=H^(H)(n)H(n) and (2) the square of the Frobenius norm of thechannel impulse response matrix H(n). The largest energy, E_(max), forall L+1 time delays is then determined as: $\begin{matrix}{E_{\max} = {\max\limits_{n = {0\quad\ldots\quad L}}{\left\{ {E(n)} \right\}.}}} & {{Eq}\quad(34)}\end{matrix}$The main path delay n_(mp) is equal to the time delay of the channelimpulse response matrix with the highest energy E_(max). The channelresponse matrix H_(mp) for the main path is then H_(mp)=H(n_(mp)).

A correlation matrix R_(mp) of H_(mp) is computed as R_(mp)=H_(mp)^(H)H_(mp). Eigenvalue decomposition of the correlation matrix R_(mp)may be expressed as:R_(mp)=V_(mp)Λ_(mp)V_(mp) ^(H)   Eq (35)where V_(mp) is a unitary matrix of eigenvectors of R_(mp) and Λ_(mp) isa diagonal matrix of eigenvalues of R_(mp).

The matrix R_(mp) is of rank N_(S) and the diagonal matrix Λ_(mp)contains N_(S) non-negative real values along the diagonal. Thefrequency-independent steering vector v_(mp) to use for eigensteering isthe principal eigenvector of R_(mp), which is the column of V_(mp) thatcorresponds to the largest eigenvalue of R_(mp).

The transmitter performs eigensteering on the data symbol stream s(n)with the steering vector v_(mp) to obtain N_(T) transmit symbol streamsχ_(mp)(n), as follows:χ_(mp)(n)=s(n)·v _(mp).   Eq (36)The spatial processing shown in equation (36) steers the transmit powerin the direction of the principal spatial channel for the strongestpropagation path.

The receiver can perform matched filtering of the received symbolstreams y _(mp)(n), which is y _(mp)(n)=H(n){circle around(×)}χ_(mp)(n)+n _(mimo)(n), in either the time domain or the frequencydomain. The time-domain matched filtering may be expressed as:{tilde over (s)}_(mp)(n)=v _(mp) ^(H) H ^(H)(L−n){circle around (×)}y_(mp)(n), for n=0, 1, . . . L.   Eq (37)The detected symbol stream {tilde over (s)}_(mp)(n) may be postprocessed (e.g., equalized) to obtain the recovered symbol streamŝ_(mp)(n).

The frequency-domain matched filtering may be expressed as:ŝ _(mp)(k)=v _(mp) ^(H) H ^(H)(k)y _(mp)(k), for k=1, 2, . . . N_(F).  Eq (38)The N_(F) recovered symbol substreams ŝ_(mp)(k), for k=1, 2, . . .N_(F), for the N_(F) subbands may be multiplexed to obtain the recoveredsymbol stream ŝ_(mp)(n).

In general, the receiver processing for main path eigensteering may beperformed similar to that described above for principal-modeeigensteering. However, the matched filtering is performed based on thesteering vector v_(mp) for the principal spatial channel of the mainpath instead of the steering vector v_(pm) for the principal spatialchannel of the “averaged” MIMO channel.

D. Receiver Eigensteering

For receiver eigensteering, the MIMO channel is viewed as being composedof N_(R) MISO channels for N_(R) receive antennas. N_(R)frequency-independent steering vectors may be obtained for the N_(R)MISO channels in similar manner as that described above for the MISOsystem.

As shown in equation (16), the matrix H(n) for the MIMO channel iscomposed of N_(R) channel impulse response vectors h _(i)(n), for i=1,2, . . . N_(R). Each row vector h _(i)(n) includes the channel impulseresponse between the N_(T) transmit antennas and receive antenna i. AnN_(T)×N_(T) correlation matrix R_(i) may be computed for each receiveantenna, as follows: $\begin{matrix}{\begin{matrix}{{\underset{\_}{R}}_{i} = {\sum\limits_{n = 0}^{L}{(n)(n)}}} \\{{= {\frac{1}{N_{F}}{\sum\limits_{k = 1}^{N_{F}}{{{\underset{\_}{h}}_{i}^{H}(k)}{\underset{\_}{h}}_{i}(k)}}}},}\end{matrix}{{{{for}\quad i} = 1},2,{\ldots\quad{N_{R}.}}}} & {{Eq}\quad(39)}\end{matrix}$Eigenvalue decomposition of the correlation matrix R_(i) for eachreceive antenna may be performed as follows:R_(i)=V_(i)Λ_(i)V_(i) ^(H) for i=1, 2, . . . N_(R),   Eq (40)where V_(i) is a unitary matrix whose columns are the eigenvectors ofR_(i); and

Λ_(i) is a diagonal matrix whose diagonal entries are the eigenvalues ofR_(i).

Since each h _(i)(n) is a row vector for one receive antenna, thecorrelation matrix R_(i) has rank of less than or equal to min {(L+1),N_(T)}. For each receive antenna i, the frequency-independent steeringvector v_(rx,i) that maximizes the received SNR for that receive antennais the column of V_(i) that corresponds to the maximum non-zeroeigenvalue of R_(i). N_(R) steering vectors v_(rx,i), for i=1, 2, . . .N_(R), are obtained for the N_(R) receive antennas and may berepresented by an N_(T)×N_(R) matrix V_(rx)=[v_(rx,1) v_(rx,2) . . .v_(rx,N) _(R) ].

One or multiple data symbol streams may be transmitted with receivereigensteering. If one data symbol stream s(n) is transmitted, then thetransmitter performs eigensteering on this data symbol stream with eachof the N_(R) steering vectors to obtain N_(T) transmit symbol streamsχ_(rx)(n), as follows: $\begin{matrix}{{{\underset{\_}{\chi}}_{rx}(n)} = {{s(n)} \cdot {\sum\limits_{i = 1}^{N_{R}}{{\underset{\_}{v}}_{{rx},i}.}}}} & {{Eq}\quad(41)}\end{matrix}$

Again, if one data symbol stream is transmitted, then the receiver canperform matched filtering of the N_(R) received symbol streams y_(rx)(n), which is y _(rx)n)=H(n){circle around (×)}χ_(rx)(n)+n_(mimo)(n), in either the time domain or the frequency domain. For thetime-domain technique, the matched filtering is first performed for eachreceive antenna, as follows:{tilde over (s)} _(rx,i)(n)=v _(rx,i) ^(H) h _(i) ^(H)(L−n){circlearound (×)} y _(mp)(n), for i=1, 2, . . . N_(R),   Eq (42)where {tilde over (s)}_(rx,i)(n) is the filtered symbol stream forreceive antenna i. The N_(R) filtered symbol streams for all N_(R)receive antennas are then combined to obtain the detected symbol stream{tilde over (s)}_(rx) (n), as follows: $\begin{matrix}{{(n)} = {\sum\limits_{i = 1}^{N_{R}}{(n).}}} & {{Eq}\quad(43)}\end{matrix}$The detected symbol stream {tilde over (s)}_(rx)(n) may bepost-processed (e.g., equalized) to obtain the recovered symbol streamŝ_(rx)(n), which is an estimate of the transmitted data symbol streams(n).

For the frequency-domain technique, the matched filtering is firstperformed for each subband of each receive antenna, as follows:ŝ _(rx,i)(k)=v _(rx,i) ^(H) h _(i) ^(H)(k) y _(rx)(k), for i=1, 2, . . .N_(R) and k=1, 2, . . . N_(F),   Eq (44)where ŝ_(rx,i)(k) is the filtered symbol substream for subband k ofreceive antenna i. The N_(R) filtered symbol substreams for all N_(R)receive antennas for subband k are then combined to obtain the detectedsymbol substream ŝ_(rx)(k) for subband k, as follows: $\begin{matrix}{{{{\hat{s}}_{rx}(k)} = {\sum\limits_{i = 1}^{N_{R}}{{\hat{s}}_{{rx},i}(k)}}},{{{for}\quad k} = 1},2,{\ldots\quad{N_{F}.}}} & {{Eq}\quad(45)}\end{matrix}$The N_(F) detected symbol substreams ŝ_(rx)(k) for all N_(F) subbandsmay be multiplexed together to obtain the recovered symbol streamŝ_(rx)(n).

If multiple (N_(D)) data symbol streams are transmitted, whereN_(S)≧N_(D)>1, then each data symbol stream may be steered to arespective set of one or more receive antennas. The transmitter performseigensteering for each data symbol stream s_(l)(n) with a set of N_(l)steering vectors for a set of N_(l) receive antennas to which that datasymbol stream is steered, where N_(l)≧1. The eigensteering at thetransmitter for each data symbol stream s_(l)(n) may be expressed as:$\begin{matrix}{{{{\underset{\_}{\chi}}_{{rx},\ell}(n)} = {(n) \cdot {\sum\limits_{j = 1}^{N_{\ell}}{\underset{\_}{v}}_{{rx},\ell,j}}}},{{{for}\quad\ell} = 1},2,{\ldots\quad N_{D}},} & {{Eq}\quad(46)}\end{matrix}$where χ_(rx,l)(n) is the N_(T) transmit symbol substreams for datasymbol stream s_(l)(n), and v_(rx,l,j), for j=1 . . . N_(l), are theN_(l) steering vectors for data symbol stream s_(l)(n).The N_(D) sets of N_(T) transmit symbol substreams for all N_(D) datasymbol streams are then combined to obtain the N_(T) transmit symbolstreams χ_(rx)(n), as follows: $\begin{matrix}{{{\underset{\_}{\chi}}_{rx}(n)} = {\sum\limits_{\ell = 1}^{N_{D}}{{{\underset{\_}{\chi}}_{{rx},\ell}(n)}.}}} & {{Eq}\quad(47)}\end{matrix}$

The receiver can perform matched filtering of the received symbol streamy_(i)(n) for each receive antenna in either the time domain as shown inequation (42) or the frequency domain as shown in equation (44). Thereceiver can then combine the filtered symbol substreams {tilde over(s)}_(l,j)(n), for j=1 . . . N_(l), from all receive antennas used foreach data symbol stream s_(l)(n) to obtain the detected symbol stream{tilde over (s)}_(l)(n) for that data symbol stream. A space-timeequalizer may be used to equalize the N_(D) detected symbol streams{tilde over (s)}_(rx)(n) to obtain N_(D) recovered symbol streamsŝ_(rx)(n).

The frequency-independent eigensteering may also be used for a MIMO-OFDMsystem. The transmitter can perform eigensteering in the time-domain, asshown in equations (20), (28), (36) and (41), where s(n) and s(n) denotesequence(s) of time-domain chips for OFDM symbols generated for the datastream(s) by OFDM modulation. The transmitter can also performeigensteering in the frequency-domain on the data symbols for eachsubband, prior to the OFDM modulation to generate OFDM symbols. Thereceiver can perform matched filtering in the time-domain, as shown inequations (22), (29), (37), (42) and (43). The receiver can also performmatched filtering in the frequency-domain, as shown in equations (23),(30), (38), (44) and (45).

iii) MISO System

FIG. 1 shows a block diagram of a transmitter 110 and a receiver 150 ina MISO system 100. At transmitter 110, a transmit (TX) data processor120 receives a data stream d(n) from a data source 112, processes (e.g.,encodes, interleaves, and modulates) the data stream in accordance witha selected transmission mode, and provides a data symbol stream s(n).The selected transmission mode may be associated with a particular datarate, a particular coding scheme or code rate, and a particularmodulation scheme to use for the data stream, which are respectivelyindicated by the data rate, coding, and modulation controls provided bya controller 140.

A TX spatial processor 130 receives the data symbol stream s(n) and mayperform wideband processing such as spectral spreading or multi-carriermodulation, as described below. TX spatial processor 130 furtherperforms eigensteering based on the frequency-independent steeringvector v_(miso) (which is also referred to as a TX steering vector)provided by controller 140. TX spatial processor 130 also multiplexespilot in with the data and provides N_(T) transmit chip streams c_(miso) (n) for the N_(T) transmit antennas. The processing by TX dataprocessor 120 and TX spatial processor 130 is described in furtherdetail below.

A transmitter unit (TMTR) 132 receives and conditions (e.g., converts toanalog, frequency upconverts, filters, and amplifies) the N_(T) transmitchip streams to obtain N_(T) modulated signals. Each modulated signal isthen transmitted from a respective transmit antenna (not shown inFIG. 1) and via the MISO channel to receiver 150. The MISO channeldistorts the transmitted signals with a channel impulse response h(n)and further degrades the transmitted signals with additive whiteGaussian noise and possibly interference from other transmissionsources.

At receiver 150, the N_(T) transmitted signals are received by a singlereceive antenna (not shown in FIG. 1), and the received signal isprovided to a receiver unit (RCVR) 154. Receiver unit 154 conditions anddigitizes the received signal to obtain a stream of samples for thetransmitted data and pilot. Receiver unit 154 provides a received symbolstream y_(miso)(n) (for the data) to a receive (RX) spatial processor160 and received pilot symbols (for the pilot) to a channel estimator172. RX spatial processor 160 performs matched filtering of the receivedsymbol stream y_(miso)(n) with a matched filter and provides a recoveredsymbol stream ŝ_(miso)(n), which is an estimate of the data symbolstream s(n) sent by transmitter 110. An RX data processor 170 thenprocesses (e.g., demodulates, deinterleaves, and decodes) the recoveredsymbol stream in accordance with the selected transmission mode toobtain a decoded data stream {circumflex over (d)}(n), which is anestimate of the data stream d(n) sent by transmitter 110. RX dataprocessor 170 may further provide the status of each received datapacket.

Channel estimator 172 processes the received pilot symbols to obtainchannel gain and SNR estimates for the MISO channel. A matrixcomputation unit 174 then processes the channel gain estimates to obtainthe frequency-independent steering vector v_(miso) for TX spatialprocessor 130 and the matched filter for RX spatial processor 160. Atransmission mode selector 176 receives the SNR estimates from channelestimator 172 and the packet status from RX data processor 170,determines a suitable transmission mode for the data stream, andprovides the selected transmission mode to a controller 180.

Controller 180 receives the steering vector v_(miso) from computationunit 174 and the selected transmission mode from transmission modeselector 176 and assembles feedback information for transmitter 110. Thefeedback information is sent to transmitter 110 and used to adjust theprocessing of the data stream d(n) sent to receiver 150. For example,transmitter 110 may use the feedback information to adjust the datarate, the coding scheme, the modulation scheme, the eigensteering, orany combination thereof, for the data stream sent to receiver 150.

Controllers 140 and 180 direct the operation at transmitter 110 andreceiver 150, respectively. Memory units 142 and 182 provide storage forprogram codes and data used by controllers 140 and 180, respectively.Memory units 142 and 182 may be internal to controllers 140 and 180, asshown in FIG. 1, or external to these controllers.

FIG. 2 shows a block diagram of an embodiment of TX data processor 120in FIG. 1. Within TX data processor 120, an encoder 212 receives andcodes the data stream d(n) based on a coding scheme indicated by thecoding control and provides code bits. The data stream may carry one ormore data packets, and each data packet is typically coded separately toobtain a coded data packet. The coding increases the reliability of thedata transmission. The coding scheme may include cyclic redundancy check(CRC) coding, convolutional coding, turbo coding, block coding, and soon, or a combination thereof. A channel interleaver 214 interleaves thecode bits based on an interleaving scheme, which may be indicated by aninterleaving control if the interleaving is dependent on transmissionmode. The interleaving provides time, frequency, and/or spatialdiversity for the code bits.

A symbol mapping unit 216 maps the interleaved bits based on amodulation scheme indicated by the modulation control and provides astream of modulation symbols (or simply, “data symbols”). Unit 216groups each set of B interleaved bits to form a B-bit binary value,where B≧1, and further maps each B-bit value to a specific modulationsymbol based on the modulation scheme (e.g., QPSK, M-PSK, or M-QAM,where M=2^(B)). Each modulation symbol is a complex value in a signalconstellation defined by the modulation scheme.

FIG. 3A shows a block diagram of a TX spatial processor 130 a, which isan embodiment of TX spatial processor 130 in FIG. 1. TX spatialprocessor 130 a includes an eigensteering unit 330, a TX pilot processor340, and a multiplexer (MUX) 350.

Eigensteering unit 330 includes N_(T) multipliers 332 a through 332 t,one multiplier 332 for each of the N_(T) transmit antennas. Eachmultiplier 332 receives the data symbol stream s(n) and a respectiveelement ν_(miso,j) of the TX steering vector v_(miso), multiplies eachdata symbol with the element ν_(miso,j), and provides a transmit symbolstream. Multipliers 332 a through 332 t perform frequency-independenteigensteering as shown in equation (5).

TX pilot processor 340 includes N_(T) multipliers 342 a through 342 t,one multiplier 342 for each of the N_(T) transmit antennas. Eachmultiplier 342 receives the pilot symbol and a unique orthogonalsequence w_(j) assigned to its transmit antenna, multiplies the pilotsymbol with the orthogonal sequence w_(j), and provides a sequence ofcovered pilot symbols. Multipliers 342 a through 342 t generate N_(T)orthogonal pilots for the N_(T) transmit antennas, which may be used forchannel estimation by receiver 150.

Multiplexer 350 includes N_(T) multiplexers 352 a through 352 t, onemultiplexer 352 for each of the N_(T) transmit antennas. Eachmultiplexer 352 receives and multiplexes the transmit symbols from anassociated multiplier 332 with the covered pilot symbols from anassociated multiplier 342 and provides a respective stream of transmitchips c_(j)(n). The pilot may be multiplexed with the data using timedivision multiplexing (TDM), as shown in FIG. 3A, code divisionmultiplexing (CDM), subband multiplexing, or some other multiplexingscheme. In any case, multiplexers 352 a through 352 t provide N_(T)transmit chip streams c_(j)(n), for j=1, 2, . . . N_(T), for the N_(T)transmit antennas.

Transmitter unit 132 includes N_(T) transmitters 362 a through 362 t,one transmitter 362 for each of the N_(T) transmit antennas. Eachtransmitter 362 receives and conditions a respective transmit chipstream to generate a modulated signal, which is then transmitted from anassociated antenna 134.

FIG. 3B shows a block diagram of a TX spatial processor 130 b, which isanother embodiment of TX spatial processor 130 in FIG. 1. TX spatialprocessor 130 b performs spectral spreading in the time domain andincludes a spreader 310, eigensteering unit 330, TX pilot processor 340,and multiplexer 350.

Within TX spatial processor 130 b, spreader 310 receives and spectrallyspreads the data symbol stream s(n) with a pseudo-random number (PN)sequence and provides a stream of spread data symbols. The spreading isespecially applicable for a low rate data symbol stream to spectrallyspread the data over the entire system bandwidth. The spreading may beperformed in similar manner as for a CDMA system, which is well known inthe art. The eigensteering is then performed on the spread data symbolstream (instead of the data symbol stream), as described above for FIG.3A, to obtain N_(T) transmit chip streams for the N_(T) transmitantennas.

FIG. 3C shows a block diagram of a TX spatial processor 130 c, which isyet another embodiment of TX spatial processor 130 in FIG. 1. TX spatialprocessor 130 c performs OFDM modulation and includes an OFDM modulator320, eigensteering unit 330, TX pilot processor 340, and multiplexer350.

Within TX spatial processor 130 c, an OFDM modulator 320 receives andperforms OFDM modulation on the data symbol stream s(n). OFDMeffectively partitions the overall system bandwidth into multiple(N_(F)) orthogonal subbands, which are also commonly referred to astones, bins, and frequency subchannels. With OFDM, each subband isassociated with a respective carrier that may be modulated with data.For each OFDM symbol period, one data or pilot symbol may be transmittedon each subband used for transmission, and a signal value of zero isprovided for each unused subband. Within OFDM modulator 320, an inversefast Fourier transform (IFFT) unit receives a set of data/pilot symbolsand zeros for the N_(F) subbands for each OFDM symbol period, transformsthe set of data/pilot symbols and zeros to the time domain using aninverse fast Fourier transform, and provides a transformed symbol thatcontains N_(F) time-domain chips. A cyclic prefix generator then repeatsa portion of each transformed symbol to obtain an OFDM symbol thatcontains N_(F)+N_(P) chips, where N_(cp) is the number of chipsrepeated. The cyclic prefix is used to combat frequency selective fadingcaused by time dispersion in the channel. OFDM modulator 320 provides astream of data chips for a stream of OFDM symbols.

Eigensteering is then performed on the data chip stream (instead of thedata symbol stream), as described above for FIG. 3A, to obtain N_(T)transmit chip streams for the N_(T) transmit antennas. Alternatively,the data symbol stream may be demultiplexed into N_(S) data symbolsubstreams and eigensteering may be performed on each data symbolsubstream. In this case, the same steering vector v_(miso) is used forall subbands. OFDM modulation may then be performed on the output of theeigensteering for all subbands of each transmit antenna to obtain thetransmit chip stream for that transmit antenna. In general, theeigensteering may be performed in either the time domain or thefrequency domain. However, eigensteering in the time domain may requirefewer multiplications and may thus be simpler to implement.

FIG. 4A shows a block diagram of an RX spatial processor 160 a, which isone embodiment of RX spatial processor 160 in FIG. 1 and may be used inconjunction with TX spatial processor 130 a in FIG. 3A. An antenna 152receives the N_(T) transmitted signals from transmitter 110 and providesthe received signal. Receiver unit 154 conditions, digitizes, andpre-processes the received signal and provides the received symbolstream y_(miso)(n). The pre-processing may include filtering,resampling, sample rate conversion, and so on.

Within RX spatial processor 160 a, a matched filter 410 performs matchedfiltering of the received symbol stream y_(miso)(n) with the matchedfilter m_(miso)(n)=v_(miso) ^(H) h ^(H)(L−n), as shown in equation (7),and provides the detected symbol stream {tilde over (s)}_(miso)(n). Anequalizer 412 then performs equalization on the detected symbol streamand provides the recovered symbol stream ŝ_(miso)(n). Equalizer 412 mayimplement an MMSE equalizer, a decision feedback equalizer, a maximumlikelihood sequence estimator, or some other type of equalizer, all ofwhich are known in the art. The equalization attempts to mitigateintersymbol interference due to frequency selectivity in the MISOchannel. The matched filtering and equalization may be integratedtogether (e.g., matched filter 410 may be embedded in equalizer 412).

FIG. 4B shows a block diagram of an RX spatial processor 160 b, which isanother embodiment of RX spatial processor 160 in FIG. 1. RX spatialprocessor 160 b performs spectral despreading in the time domain and maybe used in conjunction with TX spatial processor 130 b in FIG. 3B.Within RX spatial processor 160 b, matched filter 410 performs matchedfiltering of the received symbol stream y_(miso)(n) with the matchedfilter m_(miso)(n)=v_(miso) ^(H) h ^(H)(L−n) and provides the detectedsymbol stream {tilde over (s)}_(miso)(n). A despreader 412 thendespreads the detected symbol stream with (the complex conjugate of) thePN sequence used by transmitter 110 and provides the recovered symbolstream ŝ_(miso)(n). The despreading may be performed with a rakereceiver in similar manner as for a CDMA system, which is known in theart.

FIG. 4C shows a block diagram of an RX spatial processor 160 c, which isyet another embodiment of RX spatial processor 160 in FIG. 1. RX spatialprocessor 160 c performs OFDM demodulation and may be used inconjunction with TX spatial processor 130 c in FIG. 3C. RX spatialprocessor 160 c includes an OFDM demodulator 420, N_(F) matched filters430 a through 430 f for the N_(F) subbands, and a multiplexer 432.

Within RX spatial processor 160 c, OFDM demodulator 420 performs OFDMdemodulation on the received symbol stream y_(miso)(n). OFDM demodulator420 first removes the cyclic prefix in each received OFDM symbol toobtain a received transformed symbol. OFDM demodulator 420 thentransforms each received transformed symbol to the frequency domainusing a fast Fourier transform (FFT) to obtain a set of N_(F) receivedsymbols for the N_(F) subbands. OFDM demodulator 420 provides N_(F)received symbol substreams y_(miso)(k), for k=1, 2, . . . N_(F), for theN_(F) subbands to N_(F) matched filters 430 a through 430 f. Eachmatched filter 430 performs matched filtering of its received symbolsubstream y_(miso)(k) with its matched filter m_(miso)(k)=v_(miso) ^(H)h ^(H)(k), which is a complex-valued scalar, and provides a detectedsymbol substream {tilde over (s)}_(miso)(k). Multiplexer 432 receivesand multiplexes the N_(F) detected symbol substreams from all N_(F)matched filters 430 a through 430 f and provides the recovered symbolstream ŝ_(miso)(n). Fewer than N_(F) subbands may be used for datatransmission. In this case, the received symbols for the unused subbandsare discarded and the matched filtering is not performed for the unusedsubbands.

FIG. 5 shows a block diagram of a receiver 150 x, which is an embodimentof receiver 150 in FIG. 1. RX spatial processor 160 performs matchedfiltering and other pre-processing on the received symbol streamy_(miso)(n) and provides the recovered symbol stream ŝ_(miso)(n) to RXdata processor 170.

Within RX data processor 170, a symbol demapping unit 512 demodulatesthe recovered symbols in accordance with the modulation scheme used forthe data stream, as indicated by a demodulation control provided bycontroller 180. A channel deinterleaver 514 then deinterleaves thedemodulated data in a manner complementary to the interleaving performedat transmitter 110. If the interleaving is dependent on transmissionmode, then controller 180 provides a deinterleaving control to channeldeinterleaver 514. A decoder 516 then decodes the deinterleaved data ina manner complementary to the encoding performed at transmitter 110, asindicated by a decoding control provided by controller 180. For example,a turbo decoder or a Viterbi decoder may be used for decoder 516 iftransmitter 110 performs turbo or convolutional coding, respectively.Decoder 516 may also provide the status of each received data packet(e.g., indicating whether the packet was received correctly or inerror).

Channel estimator 172 obtains received pilot symbols from receiver unit154, estimates the MISO channel response and the noise floor at receiver150 x based on the received pilot symbols, and provides the channelimpulse response estimate ĥ(n) and the noise floor estimate {circumflexover (σ)}² to controller 180. Controller 180 performs various functionsrelated to eigensteering, matched filtering, and rate control for datatransmission. For example, a matrix computation unit 522 withincontroller 180 performs computation to derive the frequency-independentsteering vector v_(miso) for transmitter 110 and the matched filter forreceiver 150. Unit 522 may also estimate the received SNR of the datastream. A transmission mode selector 524 selects a suitable transmissionmode for the data stream d(n) based on the received SNR. Memory unit 182may store a look-up table (LUT) 526 for all of the transmission modessupported by the MISO system and their required SNRs. Controller 180provides the selected transmission mode for the data stream, the TXsteering vector, acknowledgments (ACKs) and/or negative acknowledgments(NAKs), and so on as feedback information for transmitter 110.

iv) MIMO System

For a MIMO system, N_(S) spatial channels are available for datatransmission, where N_(S)≦min {N_(T), N_(R) }. One data stream may betransmitted on each spatial channel. Each data stream may beindependently processed in accordance with a transmission mode selectedfor that data stream.

FIG. 6 shows a block diagram of a transmitter 610 and a receiver 650 ina MIMO system 600. At transmitter 610, a TX data processor 620 receivesN_(D) data streams, where N_(S)≧N_(D)≧1. TX data processor 620 codes,interleaves, and modulates each data stream in accordance with itsselected transmission mode and provides a corresponding data symbolstream. A TX spatial processor 630 receives N_(D) data symbol streamsfrom TX data processor 620, performs wideband processing (if any) andeigensteering based on a set of N_(D) or N_(R) TX steering vectorsprovided by controller 640, multiplexes in pilot, and provides N_(T)transmit chip streams for the N_(T) transmit antennas. The processing byTX data processor 620 and TX spatial processor 630 is described infurther detail below. A transmitter unit 632 receives and conditions theN_(T) transmit chip streams to obtain N_(T) modulated signals, which aretransmitted from N_(T) transmit antennas (not shown in FIG. 6) and viathe MIMO channel to receiver 650.

At receiver 650, the N_(T) transmitted signals are received by each ofN_(R) receive antennas (not shown in FIG. 6), and the N_(R) receivedsignals from the N_(R) receive antennas are provided to a receiver unit654. Receiver unit 654 conditions, digitizes, and pre-processes eachreceived signal to obtain a corresponding received symbol stream.Receiver unit 654 provides N_(R) received symbol streams to an RXspatial processor 660 and received pilot symbols to a channel estimator672. RX spatial processor 660 performs matched filtering of the N_(R)received symbol streams with N_(R) matched filters and provides N_(D)recovered symbol streams, which are estimates of the N_(D) data symbolstreams sent by transmitter 610. An RX data processor 670 then processes(e.g., demodulates, deinterleaves, and decodes) each recovered symbolstream in accordance with its transmission mode to obtain a decoded datastream, which is an estimate of the data stream sent by transmitter 610.RX data processor 670 may further provide the status of each receiveddata packet.

Channel estimator 672, matrix computation unit 674, and transmissionmode selector 676 perform similar functions as channel estimator 172,matrix computation unit 174, and transmission mode selector 176,respectively, in FIG. 1 to determine the N_(D) or N_(R) TX steeringvectors for transmitter 610, the N_(R) matched filters for receiver 650,and the N_(D) transmission modes for the N_(D) data streams. Acontroller 680 assembles feedback information for transmitter 610, whichmay include the N_(D) or N_(R) TX steering vectors and the N_(D)transmission modes.

Controllers 640 and 680 direct the operation at transmitter 610 andreceiver 650, respectively. Memory units 642 and 682 provide storage forprogram codes and data used by controllers 640 and 680, respectively.Memory units 642 and 682 may be internal to controllers 640 and 680, asshown in FIG. 6, or external to these controllers.

If N_(D)=1, then the encoding, interleaving, and modulation for thesingle data stream may be performed as shown in FIG. 2. Theeigensteering for the single data stream may be performed as shown inFIG. 3A, 3B, or 3C, depending on whether spectral spreading or OFDMmodulation is performed on the data stream. However, the eigensteeringis performed with the steering vector v_(pm) for the principal mode orthe steering vector v_(mp) for the main path (instead of the steeringvector v_(miso)). The receiver matched filtering may be performed asdescribed below. If N_(D)>1, then the data processing (e.g., encoding,interleaving, and modulation) and the eigensteering may be performed asdescribed below.

FIG. 7 shows a block diagram of an embodiment of TX data processor 620in FIG. 6. For this embodiment, TX data processor 620 includes one setof encoder 712, channel interleaver 714, and symbol mapping unit 716 foreach of the N_(D) data streams. Each set of encoder, channelinterleaver, and symbol mapping unit receives and processes a respectivedata stream d_(l)(n) in similar manner as described above for TX dataprocessor 120 in FIG. 2 to obtain a corresponding data symbol streams_(l)(n). The coding, interleaving, and modulation for each data streamare performed based on the coding, interleaving, and modulation controlsprovided by controller 640, which are generated based on thetransmission mode selected for that data stream.

FIG. 8A shows a block diagram of a TX spatial processor 630 a, which isan embodiment of TX spatial processor 630 in FIG. 6 and may be used formulti-mode eigensteering. For this embodiment, TX spatial processor 630a includes N_(D) eigensteering units 830 a through 830 d for the N_(D)data streams, a TX pilot processor 840, a combiner 850, and amultiplexer 860.

For multi-mode eigensteering, each eigensteering unit 830 receives arespective data symbol stream s_(l)(n) and a respectivefrequency-independent steering vector v_(l) in the matrix V_(mm). Eacheigensteering unit 830 performs eigensteering on its data symbol streamwith its steering vector, as described above for FIG. 3A, and provides arespective set of N_(T) transmit symbol substreams χ_(l)(n) for theN_(T) transmit antennas. Eigensteering units 830 a through 830 d provideN_(D) sets of transmit symbol substreams for the N_(D) data streams.Combiner 850 includes N_(T) combiners 852 a through 852 t, one combiner852 for each of the N_(T) transmit antennas. Each combiner 852 receivesand combines a respective set of N_(D) transmit symbol substreams fromeigensteering units 830 a through 830 d for its transmit antenna andprovides a transmit symbol stream. Combiners 852 a through 852 t provideN_(T) transmit symbol streams χ_(mm)(n) for the N_(T) transmit antennas.Eigensteering units 830 a through 830 d and combiner 850 collectivelyperform the eigensteering shown in equation (28).

For receiver eigensteering, TX spatial processor 630 a would includeN_(R) eigensteering units 830, one for each of the N_(R) receiveantennas. Each eigensteering unit 830 would receive a respectivefrequency-independent steering vector v_(rx,i) in the matrix V_(rx). IfN_(D)=1, then the same data symbol stream s(n) is provided to all N_(R)eigensteering units and steered with the N_(R) steering vectors toobtain N_(R) sets of N_(T) transmit symbol substreams. Each combiner 852would then receive and combine a respective set of N_(R) transmit symbolsubstreams from the N_(R) eigensteering units for its transmit antennaand provides a transmit symbol stream. If N_(D)>1, then each data symbolstream may be provided to one or more of the N_(R) eigensteering unitsfor the one or more receive antennas to which the data symbol stream issteered. The eigensteering is then performed in similar manner to obtainthe N_(T) transmit symbol streams χ_(rx)(n) for the N_(T) transmitantennas.

TX pilot processor 840 receives and covers the pilot symbol with N_(T)orthogonal sequences, as described above for FIG. 3A, and provides N_(T)sequences of covered pilot symbols for the N_(T) transmit antennas.Multiplexer 860 includes N_(T) multiplexers 862 a through 862 t, onemultiplexer 862 for each of the N_(T) transmit antennas. Eachmultiplexer 862 receives and multiplexes the transmit symbols from anassociated combiner 852 with the covered pilot symbols from anassociated multiplier 842 and provides a respective stream of transmitchips. Multiplexers 862 a through 862 t provide N_(T) transmit chipstreams c _(mimo)(n)=[c₁(n) c₂(n) . . . c_(N) _(T) (n)]^(T) for theN_(T) transmit antennas.

FIG. 8B shows a block diagram of a TX spatial processor 630 b, which isanother embodiment of TX spatial processor 630 in FIG. 6. TX spatialprocessor 630 b performs spectral spreading in the time domain andincludes N_(D) spreaders 810 a through 810 d for N_(D) data streams,N_(D) eigensteering units 830 a through 830d, TX pilot processor 840,combiner 850, and multiplexer 860. Each spreader 810 receives andspectrally spreads a respective data symbol stream s_(l)(n) with a PNspreading sequence and provides a corresponding stream of spread datasymbols. The same or different PN sequences may be used for the N_(D)data symbol streams. Spreaders 810 a through 810 d provide N_(D) spreaddata symbol streams for the N_(D) data symbol streams. Eigensteering isthen performed on each of the N_(D) spread data symbol streams (insteadof the data symbol streams), in similar manner as described above forFIGS. 3A and 8A, to obtain N_(T) transmit chip streams for the N_(T)transmit antennas.

FIG. 8C shows a block diagram of a TX spatial processor 630 c, which isyet another embodiment of TX spatial processor 630 in FIG. 6. TX spatialprocessor 630 c performs OFDM modulation and includes N_(D) OFDMmodulators 820 a through 820 d for N_(D) data streams, N_(D)eigensteering units 830 a through 830 d, TX pilot processor 840,combiner 850, and multiplexer 860.

Each OFDM modulator 820 performs OFDM modulation on a respective datasymbol stream s_(l)(n), in similar manner as described above for FIG.3C, and provides a stream of data chips. OFDM modulators 820 a through820 d provide N_(D) data chip streams for the N_(D) data streams.Eigensteering is then performed on each of the N_(D) data chip streams(instead of the data symbol stream), as described above for FIGS. 3A and8A, to obtain N_(T) transmit chip streams for the N_(T) transmitantennas. Alternatively, the eigensteering may be performed in thefrequency domain on the data symbol substream for each subband. In thiscase, each eigensteering unit uses the same steering vector v_(l) forall subbands.

FIG. 9A shows a block diagram of an RX spatial processor 660 a, whichmay be used for the case in which a single data stream is transmitted(i.e., N_(D)=1). Each of N_(R) receive antennas 652 a through 652 rreceives the N_(T) transmitted signals from transmitter 610 and providesa received signal to an associated receiver unit 654. Each receiver unit654 conditions, digitizes, and pre-processes its received signal andprovides a received symbol stream y_(i)(n).

-   -   1. RX spatial processor 660 a includes N_(R) matched filters 910        a through 910 r for the N_(R) receive antennas, a combiner 912,        and an equalizer 914. Each matched filter 910 performs matched        filtering of its received symbol stream y_(i)(n) with a matched        filter m_(i)(n)=v_(mimo) ^(H) h _(i) ^(H)(L−n) and provides a        filtered symbol stream.

The vector v_(mimo) is equal to the steering vector v_(pm) for principalmode eigensteering, the steering vector v_(mp) for main patheigensteering, or the steering vector v_(rx,i) for receivereigensteering. For receiver eigensteering, each matched filter 910 isassociated with a different steering vector v_(rx,i) for its receiveantenna, which is not shown in FIG. 9A. The vector h _(i)(n) is thechannel impulse response between the N_(T) transmit antennas and receiveantenna i. Combiner 912 receives and combines the N_(R) filtered symbolstreams from matched filters 910 a through 910 r and provides a detectedsymbol stream {tilde over (s)}_(mimo)(n). Equalizer 914 performsequalization on the detected symbol stream and provides the recoveredsymbol stream ŝ_(mimo)(n). Equalizer 914 may implement an MMSEequalizer, a decision feedback equalizer, a maximum likelihood sequenceestimator, or some other type of equalizer.

FIG. 9B shows a block diagram of an RX spatial processor 660 b, whichmay also be used for the case in which a single data stream istransmitted (i.e., N_(D)=1). RX spatial processor 660 b performsspectral despreading in the time domain and may be used in conjunctionwith TX spatial processor 630 b in FIG. 8B.

RX spatial processor 660 b includes N_(R) matched filters 910 a through910 r for the N_(R) receive antennas, combiner 912, and a despreader916. Each matched filter 910 performs matched filtering of a respectivereceived symbol stream y_(i)(n) with its matched filterm_(i)(n)=v_(mimo) ^(H) h _(i) ^(H)(L−n) and provides a filtered symbolstream. Combiner 912 receives and combines the N_(R) filtered symbolstreams and provides the detected symbol stream ŝ_(mimo)(n). Despreader916 then despreads the detected symbol stream with the PN sequence usedby transmitter 610 and provides the recovered symbol stream ŝ_(mimo)(n).

FIG. 9C shows a block diagram of an RX spatial processor 660 c, whichmay also be used for the case in which a single data stream istransmitted (i.e., N_(D)=1). RX spatial processor 660 c performs OFDMdemodulation and may be used in conjunction with TX spatial processor630 c in FIG. 8C.

RX spatial processor 660 c includes N_(R) antenna matched filters 920 athrough 920 r for the N_(R) receive antennas, N_(F) combiners 932 athrough 932f for the N_(F) subbands, and a multiplexer 934. Each antennamatched filter 920 performs matched filtering for one receive antennaand includes an OFDM demodulator 922 and N_(F) matched filters 930 athrough 930 f for the N_(F) subbands.

Within each antenna matched filter 920, OFDM demodulator 922 performsOFDM demodulation on the received symbol stream y_(i)(n) for theassociated receive antenna and provides N_(F) received symbol substreamsy_(i)(k), for k=1, 2, . . . N_(F), for the N_(F) subbands to N_(F)matched filters 930 a through 930 f. Each matched filter 930 performsmatched filtering of its received symbol substream y_(i)(k) with itsmatched filter m_(i)(k)=v_(mimo) ^(H) h _(i) ^(H)(k) and provides afiltered symbol substream. The vector h _(i)(k) is the channel frequencyresponse for receive antenna i for subband k. Matched filters 930 athrough 930 f for each antenna matched filter 920 provide N_(F) filteredsymbol substreams for the N_(F) subbands to N_(F) combiners 932 athrough 932 f.

Each combiner 932 receives and combines the N_(R) filtered symbolsubstreams from N_(R) antenna matched filters 920 a through 920 r forits subband and provides a detected symbol substream for the subband.Multiplexer 934 receives and multiplexes the N_(F) detected symbolsubstreams from combiners 932 a through 932 f for the N_(F) subbands andprovides the recovered symbol stream ŝ_(mimo)(n).

FIG. 9D shows a block diagram of an RX spatial processor 660 d, whichmay be used for multi-mode eigensteering with N_(D)>1. RX spatialprocessor 660 d may be used in conjunction with TX spatial processor 630a in FIG. 8A or TX spatial processor 630 b in FIG. 8B.

RX spatial processor 660 d includes N_(R) matched filters 940 a through940r for the N_(R) receive antennas, N_(D) combiners 942 a through 942 dfor the N_(D) data streams, a space-time equalizer 944, and N_(D)despreaders 946 a through 946 d for the N_(D) data streams. Each matchedfilter 940 performs matched filtering of a respective received symbolstream y_(i)(n) with a matched filter m _(i)(k)=V_(mm) ^(H) h _(i)^(H)(L−n) for an associated receive antenna and provides N_(D) filteredsymbol substreams for the N_(D) data symbol streams. The matrix V_(mm)includes N_(D) steering vectors v_(l), for l=1, 2, . . . N_(D), for theN_(D) data symbol streams. Each matched filter 940 thus performs matchedfiltering of the received symbol stream y_(i)(n) with N_(D) matchedfilters m_(i,l)(n)=v_(l) ^(H) h _(i) ^(H)(L−n), for l=1, 2, . . . N_(D),where v_(l) is the l-th column of V_(mm), to obtain the N_(D) filteredsymbol substreams for the associated receive antenna.

Each combiner 942 receives and combines the N_(R) filtered symbolsubstreams from matched filters 940 a through 940 r for one data symbolstream and provides the detected symbol stream {tilde over (s)}_(l)(n)for the data stream. Matched filters 940 a through 940 r and combiners942 a through 942 d collectively perform the matched filtering shown inequation (29) and provides N_(D) detected symbol streams {tilde over(s)}(n) for the N_(D) data symbol streams.

If multiple data symbol streams are transmitted, then there is likely tobe cross-talk between these data symbol streams at receiver 650.Space-time equalizer 944 performs equalization on the N_(D) detectedsymbol streams from combiners 942 a through 942 d and provides N_(D)equalized symbol streams. Space-time equalizer 944 may implement an MMSElinear equalizer, a decision feedback equalizer, a maximum likelihoodsequence estimator, or some other type of equalizer that can jointlyoperate on multiple streams to mitigate cross-talk and/or maximize thereceived SNR in the presence of cross-talk, intersymbol interference,and noise. Space-time equalizer 944 may also implement the successiveequalization and interference cancellation processing technique.Space-time equalizer 944 may also be omitted.

If spreading is not performed at transmitter 610, as shown in FIG. 8A,then the N_(D) equalized symbol streams from space-time equalizer 944are provided as the N_(D) recovered symbol streams ŝ(n). If spreading isperformed at transmitter 610 for each data symbol stream, as shown inFIG. 8B, then each despreader 946 receives and despreads a respectiveequalized symbol stream with the PN sequence and provides acorresponding recovered symbol stream. Despreaders 946 a through 946 dwould then provide the N_(D) recovered symbol streams ŝ(n).

FIG. 9E shows a block diagram of an RX spatial processor 660 e, whichmay be used for receiver eigensteering with N_(D)>1. RX spatialprocessor 660 e includes N_(R) matched filters 950 a through 950 r forthe N_(R) receive antennas, a combiner 952, a space-time equalizer 954,and N_(D) despreaders 956 a through 956 d for the N_(D) data symbolstreams. Each matched filter 950 performs matched filtering of arespective received symbol stream y_(i)(n) with a matched filter m_(rx,i)(k)=v_(rx,i) ^(H) h _(i) ^(H)(L−n) for an associated receiveantenna and provides a filtered symbol stream. Combiner 952 receives theN_(R) filtered symbol streams from matched filters 950 a through 950 r,combines the filtered symbol streams for all receive antennas used foreach data symbol stream, and provides the detected symbol stream {tildeover (s)}_(l)(n) for that data symbol stream. The combining is dependenton the eigensteering performed at the transmitter (i.e., the specificreceive antennas to which each data symbol stream is steered). Combiner952 provides N_(D) detected symbol streams {tilde over (s)}(n) for theN_(D) data symbol streams. Space-time equalizer 954 and despreaders 956a through 956 d operate on the N_(D) detected symbol streams asdescribed above for FIG. 9D and provide the N_(D) recovered symbolstreams ŝ(n).

FIG. 9F shows a block diagram of an RX spatial processor 660 f, whichmay also be used for multi-mode eigensteering with N_(D)>1. RX spatialprocessor 660 f performs OFDM demodulation and may be used inconjunction with TX spatial processor 630 c in FIG. 8C.

RX spatial processor 660 f includes N_(R) antenna matched filters 970 athrough 970 r for the N_(R) receive antennas, N_(F) combiners 982 athrough 982f for the N_(F) subbands, N_(F) space-time equalizers 984 athrough 984 f for the N_(F) subbands, and a multiplexer 986. Eachantenna matched filter 970 performs matched filtering for one receiveantenna and includes an OFDM demodulator 972 and N_(F) matched filters980 a through 980 f for the N_(F) subbands.

Within each antenna matched filter 970, OFDM demodulator 972 performsOFDM demodulation on the received symbol stream y_(i)(n) for theassociated antenna and provides N_(F) received symbol substreamsy_(i)(k), for k=1, 2, . . . N_(F), for the N_(F) subbands to N_(F)matched filters 980 a through 980 f. Each matched filter 980 performsmatched filtering of its received symbol substream y_(i)(k) with itsmatched filter m _(i)(k)=V_(mm) ^(H) h _(i) ^(H)(k) and provides N_(D)filtered symbol substreams for the N_(D) data symbol streams for itssubband. Matched filters 980 a through 980 f for each antenna matchedfilter 970 provide N_(F) sets of N_(D) filtered symbol substreams forthe N_(F) subbands to N_(F) combiners 982 a through 982 f.

Each combiner 982 receives and combines the N_(R) sets of N_(D) filteredsymbol substreams from N_(R) antenna matched filters 970 a through 970 rfor its subband and provides N_(D) detected symbol substreams for itssubband. Although not shown in FIG. 9F, each combiner 982 includes N_(D)summers, one summer for each data symbol stream. Each summer receivesand sums the N_(R) filtered symbol substreams from antenna matchedfilters 970 a through 970 r for its subband and its data symbol streamto obtain the detected symbol substream for its subband.

Each spatial equalizer 984 performs equalization on the N_(D) detectedsymbol substreams from an associated combiner 982 for its subband andprovides N_(D) equalized symbol streams for the subband. Spatialequalizer 984 may implement an MMSE linear equalizer or some otherequalizer that jointly operates on multiple symbol streams to mitigatecross-talk and/or maximize the received SNR. Spatial equalizer 984 mayalso implement the successive equalization and interference cancellationprocessing technique.

Multiplexer 986 receives N_(F) sets of N_(D) equalized symbol substreamsfrom combiners 984 a through 984 f for the N_(F) subbands. Multiplexer986 then multiplexes the N_(F) equalized symbol substreams fromcombiners 984 a through 984 f for each data symbol stream and providesthe recovered symbol stream ŝ_(l)(n) for that data symbol stream.

FIG. 10 shows a block diagram of a receiver 650 x, which is anembodiment of receiver 650 in FIG. 6. RX spatial processor 660 performsmatched filtering and post-processing on the N_(R) received symbolstreams y_(i)(n), for i=1, 2, . . . N_(R), and provides N_(D) recoveredsymbol streams ŝ_(l)(n), for l=1, 2, . . . N_(D), to RX data processor670. RX data processor 670 includes one set of symbol demapping unit1012, deinterleaver 1014, and decoder 1016 for each of the N_(D)recovered symbol streams. Each set of symbol demapping unit,deinterleaver, and decoder processes a respective recovered symbolstream as described above for FIG. 5. RX data processor 670 providesN_(D) decoded data streams {circumflex over (d)}_(l)(n), for l=1, 2, . .. N_(D).

Channel estimator 672 estimates the channel response and the receivernoise floor based on the received pilot symbols from receiver units 654a through 654 r and provides the channel impulse response estimate Ĥ(n)and the noise floor estimate {circumflex over (σ)}² to controller 680.Controller 680 performs various functions related to eigensteering,matched filtering, and rate control for the data transmission. Forexample, a matrix computation unit 1022 may perform computation toderive (1) the steering vector v_(pm) for principal mode eigensteering,(2) the steering vector v_(mp) for main path eigensteering, (3) N_(D)steering vectors v_(l), for l=1, 2, . . . N_(D), for multi-modeeigensteering, or (4) N_(R) steering vectors v_(rx,i), for i=1, 2, . . .N_(R), for receiver eigensteering. Computation unit 1022 alsoderives_N_(R) matched filters for receiver 650 and may further estimatethe received SNR of the N_(D) data streams. A transmission mode selector1024 selects a suitable transmission mode for each data stream based onits received SNR. Memory unit 682 may store a look-up table 1026 for allof the supported transmission modes and their required SNRs. Controller680 provides the N_(D) TX steering vectors, the N_(D) selectedtransmission modes for the N_(D) data streams, ACKs and/or NAKs, and soon as feedback information for transmitter 610.

For embodiments described above, the receiver estimates the channelresponse of the MISO or MIMO channel, derives the steering vector(s) forthe transmitter and the matched filter(s) for the receiver, and sendsback the steering vector(s) as feedback information. For otherembodiments, it may be possible for the transmitter to estimate thechannel response and derive the steering vector(s). For example, in atime division duplexed (TDD) system with a shared frequency band, thedownlink and uplink channel responses may be assumed to be reciprocal ofone another. That is, if H(k) represents a channel frequency responsematrix from antenna array A to antenna array B for subband k, then areciprocal channel implies that the coupling from array B to array A isgiven by H^(T)(k). For the TDD system, the reciprocal channelcharacteristics can be exploited to allow the transmitter to estimatethe link observed by the receiver based on pilot sent by the receiver onthe other link. In general, the channel estimation and the computationof the steering vectors may be performed by the receiver or thetransmitter, depending on the system design.

FIG. 11 shows a flow diagram of an embodiment of a process 1100 forperforming principal mode eigensteering, multi-mode eigensteering, andmain path eigensteering in a MIMO system. Initially, a plurality ofchannel response matrices are obtained for the channel response of theMIMO channel in the MIMO system (at block 1112). These channel responsematrices may be (1) L+1 channel impulse response matrices for L+1 timedelays (i.e., H(n) for n=0, 1, . . . L) or (2) N_(F) channel frequencyresponse matrices for N_(F) subbands (i.e., H(k) for k=1, 2, . . .N_(F))

A single correlation matrix is computed for the MIMO channel based onthe channel response matrices (at block 1114). For principal modeeigensteering and multi-mode eigensteering, the single correlationmatrix may be obtained by (1) computing a correlation matrix of each ofthe plurality of channel response matrices and (2) summing thecorrelation matrices for the channel response matrices to obtain thesingle correlation matrix, as shown in equation (18). For main patheigensteering, the single correlation matrix may be obtained by (1)determining the energy of each of the channel impulse response matrices,(2) identifying the channel impulse response matrix with the highestenergy, (3) computing a correlation matrix of the channel impulseresponse matrix with the highest energy, and (4) defining the singlecorrelation matrix for the MIMO channel as the correlation matrix of thechannel impulse response matrix with the highest energy, as shown inequations (33) and (34) and the related description.

The single correlation matrix is then decomposed (e.g., using eigenvaluedecomposition) to obtain N_(D) steering vectors for N_(D) spatialchannels of the MIMO channel, where N_(S)≧N_(D)≧1 and N_(S) is thenumber of eigenmodes of the single correlation matrix (at block 1116).For principal mode eigensteering and main path eigensteering, N_(D)=1and only one steering vector is obtained. For multi-mode eigensteering,N_(D)>1 and multiple steering vectors are obtained.

The operations shown at blocks 1112, 1114, and 1116 may be performed byreceiver 650 in FIG. 6. The operations shown at blocks 1112, 1114, and1116 may also be performed by transmitter 610 for a time-divisionduplexed (TDD) system in which the downlink and uplink share the samefrequency band. In any case, the N_(D) steering vectors may be used foreigensteering by the transmitter and matched filtering by the receiver.

At the transmitter, each steering vector may be used forfrequency-independent eigensteering or spatial processing of a datastream sent on the spatial channel associated with the steering vector(at block 1122). The transmitter performs eigensteering on N_(D) datasymbol streams with the N_(D) steering vectors to generate N_(T)transmit symbol streams (at block 1124), which are further processed andtransmitted from the N_(T) transmit antennas (at block 1126).

At the receiver, matched filtering of N_(R) received symbol streams forN_(R) receive antennas, where N_(R)≧N_(D), may be performed in eitherthe time domain or frequency domain. A matched filter may be derived foreach receive antenna based on the N_(D) steering vectors and a pluralityof channel response vectors for that receive antenna (at block 1132).The channel response vectors for each receive antenna may be obtainedfrom the channel response matrices. The received symbol stream for eachreceive antenna is filtered with the matched filter for that receiveantenna to obtain N_(D) filtered symbol substreams, one substream foreach steering vector used by the transmitter (at block 1134). Thefiltered symbol substreams from all N_(R) matched filters for the N_(R)receive antennas are then combined to obtain N_(D) detected symbolstreams for the N_(D) data streams sent by the transmitter (at block1136). Equalization may be performed on the N_(D) detected symbolstreams to obtain N_(D) recovered symbol streams (at block 1138). IfN_(D)>1, then space-time equalization (e.g., with an MMSE-LE, a DFE, oran MLSE) may be performed on the multiple detected symbol streams toobtain multiple recovered symbol streams.

FIG. 12 shows a flow diagram of an embodiment of a process 1200 forperforming receiver eigensteering in a MISO or MIMO system with N_(T)transmit antennas and N_(R) receive antennas, where N_(T)>1 and N_(R)≧1in this case. Initially, N_(R) sets of channel response vectors areobtained for the N_(R) receive antennas, one set for each receiveantenna (at block 1212). Each set of channel response vectors isindicative of the channel frequency response or the channel impulseresponse between the N_(T) transmit antennas and one receive antenna.

A single correlation matrix is computed for each receive antenna basedon the set of channel response vectors for that receive antenna (atblock 1214). This may be achieved by (1) computing a correlation matrixof each of the channel response vectors for the receive antenna and (2)summing correlation matrices for the channel response vectors for thereceive antenna to obtain the single correlation matrix for the receiveantenna. The single correlation matrix for each receive antenna is thendecomposed (e.g., using eigenvalue decomposition) to obtain a steeringvector for the receive antenna (at block 1216). The operations shown atblocks 1212, 1214, and 1216 may be performed by receiver 150 in FIG. 1or receiver 650 in FIG. 6. The operations shown at blocks 1212, 1214,and 1216 may also be performed by transmitter 110 in FIG. 1 ortransmitter 610 in FIG. 6 for a TDD system. In any case, N_(R) steeringvectors are obtained for the N_(R) receive antennas and may be used forspatial processing by the transmitter and matched filtering by thereceiver.

At the transmitter, the N_(R) steering vectors may be used forfrequency-independent eigensteering or spatial processing of N_(D) datastreams, where N_(R)≧N_(D)>1 and N_(T)≧N_(D) (at block 1222). For a MISOsystem with only one receive antenna ( N_(R)=1), one data stream is sentusing one steering vector obtained for the one receive antenna (atblocks 1224 and 1226). For a MIMO system with a plurality of receiveantennas (N_(R)>1), one or multiple data streams may be sent using theN_(R) steering vectors obtained for the N_(R) receive antennas. Eachdata stream may be steered toward one or multiple receive antennas.

At the receiver, matched filtering of N_(R) received symbol streams forthe N_(R) receive antennas may be performed in either the time domain orthe frequency domain. A matched filter is derived for each receiveantenna based on the steering vector and the set of channel responsevectors for that receive antenna (at block 1232). The received symbolstream for each receive antenna is filtered with the matched filter forthat receive antenna to obtain a filtered symbol stream for the receiveantenna (at block 1234). The N_(R) filtered symbol streams from theN_(R) matched filters for the N_(R) receive antennas are then combinedto obtain N_(D) detected symbol streams for the N_(D) data streams sentby the transmitter (at block 1236). Equalization may be performed on theN_(D) detected symbol streams to obtain N_(D) recovered symbol streamsfor the N_(D) data streams sent by the transmitter (at block 1238).

v) Rate Selection

For both MISO system 100 and MIMO system 600, the receiver may estimatethe received SNR for each spatial channel. The SNR computation may bedependent on the eigensteering scheme used for data transmission, asdescribed above. The receiver may then compute an operating SNR,γ_(op)(l), for each spatial channel based on the received SNR, γ_(rx)(l)and an SNR offset, γ_(os)(l) for the spatial channel (e.g.,γ_(op)(l)=γ_(rx)(l)+γ_(os)(l) where the units are in dB). The SNR offsetmay be used to account for estimation error, variability in the channel,and other factors. The receiver may select a suitable transmission modefor each spatial channel based on the operating SNR for that spatialchannel.

The system may be designed to support a set of transmission modes. Oneof the supported transmission modes may be for a null rate (i.e., a datarate of zero). Each of the remaining transmission modes is associatedwith a particular non-zero data rate, a particular coding scheme or coderate, a particular modulation scheme, and a particular minimum SNRrequired to achieve the desired level of performance (e.g., 1% packeterror rate (PER)) for a non-fading AWGN channel. For each supportedtransmission mode with a non-zero data rate, the required SNR isobtained based on the specific system design (i.e., the particular coderate, interleaving scheme, modulation scheme, and so on, used by thesystem for that transmission mode) and for an AWGN channel. The requiredSNR may be obtained by computer simulation, empirical measurements, andso on, as is known in the art. The set of supported transmission modesand their required SNRs may be stored in a look-up table.

The operating SNR, γ_(op)(l), for each spatial channel may be providedto the look-up table, which then provides the transmission mode q(l) forthat spatial channel. This transmission mode q(l) is the supportedtransmission mode with the highest data rate and a required SNR,γ_(req)(l), that is less than or equal to the operating SNR (i.e.,γ_(req)(l)≦γ_(op)(l) ). The receiver thus selects the highest possibledata rate for each spatial channel based on the operating SNR for thatspatial channel.

For clarity, specific embodiments of various eigensteering schemes havebeen described above. Other variants of these eigensteering schemes mayalso be devised, and this is within the scope of the invention. Forexample, the single correlation matrix for the MIMO channel may becomputed in other manners than that described above for the principalmode and multi-mode eigensteering schemes. As another example, multipledata symbol streams may be transmitted on multiple spatial channels ofthe main path. As yet another example, N_(D) data symbol streams may betransmitted on the N_(D) best spatial channels based on the energy ofthe spatial channels. Other eigensteering schemes may also be devisedbased on the teachings provided herein, and this is within the scope ofthe invention.

The eigensteering techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, software, or a combination thereof. For a hardwareimplementation, the processing at the transmitter for eigensteering andother pertinent functions may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof. The processing at the receiver formatched filtering and other pertinent functions may also be implementedwithin one or more ASICs, DSPs, and so on.

For a software implementation, the eigensteering techniques may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin memory units (e.g., memory units 142 and 182 in FIG. 1 or memoryunits 642 and 682 in FIG. 6) and executed by a processor (e.g.,controllers 140 and 180 in FIG. 1 or controller 640 and 680 in FIG. 6).The memory unit may be implemented within the processor or external tothe processor, in which case it can be communicatively coupled to theprocessor via various means as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of performing spatial processing in a wireless multiple-input multiple-output (MIMO) communication system, comprising: obtaining a plurality of channel response matrices for a channel response of a MIMO channel in the MIMO system; computing a correlation matrix for the MIMO channel based on the plurality of channel response matrices; and decomposing the correlation matrix to obtain at least one steering vector for at least one spatial channel of the MIMO channel, wherein the at least one steering vector is used by a transmitting entity for frequency-independent spatial processing of a data stream sent on the at least one spatial channel associated with the at least one steering vector. 